diff --git a/Classes/PHPExcel.php b/Classes/PHPExcel.php
index 188ed258..8f89c1bc 100644
--- a/Classes/PHPExcel.php
+++ b/Classes/PHPExcel.php
@@ -208,12 +208,9 @@ class PHPExcel
      */
     public function addSheet(PHPExcel_Worksheet $pSheet = null, $iSheetIndex = null)
     {
-        if(is_null($iSheetIndex))
-        {
+        if($iSheetIndex === NULL) {
             $this->_workSheetCollection[] = $pSheet;
-        }
-        else
-        {
+        } else {
             // Insert the sheet at the requested index
             array_splice(
                 $this->_workSheetCollection,
@@ -226,7 +223,6 @@ class PHPExcel
 			if ($this->_activeSheetIndex >= $iSheetIndex) {
 				++$this->_activeSheetIndex;
 			}
-
         }
 		return $pSheet;
     }
@@ -417,7 +413,7 @@ class PHPExcel
 	 * @return PHPExcel_Worksheet
 	 */
 	public function addExternalSheet(PHPExcel_Worksheet $pSheet, $iSheetIndex = null) {
-		if (!is_null($this->getSheetByName($pSheet->getTitle()))) {
+		if ($this->getSheetByName($pSheet->getTitle()) !== NULL) {
 			throw new Exception("Workbook already contains a worksheet named '{$pSheet->getTitle()}'. Rename the external sheet first.");
 		}
 
@@ -477,14 +473,14 @@ class PHPExcel
 	public function getNamedRange($namedRange, PHPExcel_Worksheet $pSheet = null) {
 		$returnValue = null;
 
-		if ($namedRange != '' && !is_null($namedRange)) {
+		if ($namedRange != '' && ($namedRange !== NULL)) {
 			// first look for global defined name
 			if (isset($this->_namedRanges[$namedRange])) {
 				$returnValue = $this->_namedRanges[$namedRange];
 			}
 
 			// then look for local defined name (has priority over global defined name if both names exist)
-			if (!is_null($pSheet) && isset($this->_namedRanges[$pSheet->getTitle() . '!' . $namedRange])) {
+			if (($pSheet !== NULL) && isset($this->_namedRanges[$pSheet->getTitle() . '!' . $namedRange])) {
 				$returnValue = $this->_namedRanges[$pSheet->getTitle() . '!' . $namedRange];
 			}
 		}
@@ -500,7 +496,7 @@ class PHPExcel
 	 * @return PHPExcel
 	 */
 	public function removeNamedRange($namedRange, PHPExcel_Worksheet $pSheet = null) {
-		if (is_null($pSheet)) {
+		if ($pSheet === NULL) {
 			if (isset($this->_namedRanges[$namedRange])) {
 				unset($this->_namedRanges[$namedRange]);
 			}
diff --git a/Classes/PHPExcel/CachedObjectStorage/CacheBase.php b/Classes/PHPExcel/CachedObjectStorage/CacheBase.php
index 28353387..7a09ff77 100644
--- a/Classes/PHPExcel/CachedObjectStorage/CacheBase.php
+++ b/Classes/PHPExcel/CachedObjectStorage/CacheBase.php
@@ -221,7 +221,7 @@ class PHPExcel_CachedObjectStorage_CacheBase {
 	 */
 	public function copyCellCollection(PHPExcel_Worksheet $parent) {
 		$this->_parent = $parent;
-		if ((!is_null($this->_currentObject)) && (is_object($this->_currentObject))) {
+		if (($this->_currentObject !== NULL) && (is_object($this->_currentObject))) {
 			$this->_currentObject->attach($parent);
 		}
 	}	//	function copyCellCollection()
diff --git a/Classes/PHPExcel/CachedObjectStorageFactory.php b/Classes/PHPExcel/CachedObjectStorageFactory.php
index 12e60c5c..e29f5489 100644
--- a/Classes/PHPExcel/CachedObjectStorageFactory.php
+++ b/Classes/PHPExcel/CachedObjectStorageFactory.php
@@ -66,7 +66,7 @@ class PHPExcel_CachedObjectStorageFactory {
 
 
 	public static function getCacheStorageMethod() {
-		if (!is_null(self::$_cacheStorageMethod)) {
+		if (self::$_cacheStorageMethod !== NULL) {
 			return self::$_cacheStorageMethod;
 		}
 		return null;
@@ -74,7 +74,7 @@ class PHPExcel_CachedObjectStorageFactory {
 
 
 	public static function getCacheStorageClass() {
-		if (!is_null(self::$_cacheStorageClass)) {
+		if (self::$_cacheStorageClass !== NULL) {
 			return self::$_cacheStorageClass;
 		}
 		return null;
@@ -115,7 +115,7 @@ class PHPExcel_CachedObjectStorageFactory {
 			}
 		}
 
-		if (is_null(self::$_cacheStorageMethod)) {
+		if (self::$_cacheStorageMethod === NULL) {
 			self::$_cacheStorageClass = 'PHPExcel_CachedObjectStorage_'.$method;
 			self::$_cacheStorageMethod = $method;
 		}
@@ -125,13 +125,13 @@ class PHPExcel_CachedObjectStorageFactory {
 
 	public static function getInstance(PHPExcel_Worksheet $parent) {
 		$cacheMethodIsAvailable = TRUE;
-		if (is_null(self::$_cacheStorageMethod)) {
+		if (self::$_cacheStorageMethod === NULL) {
 			$cacheMethodIsAvailable = self::initialize();
 		}
 
 		if ($cacheMethodIsAvailable) {
 			$instance = new self::$_cacheStorageClass($parent,self::$_storageMethodParameters[self::$_cacheStorageMethod]);
-			if (!is_null($instance)) {
+			if ($instance !== NULL) {
 				return $instance;
 			}
 		}
diff --git a/Classes/PHPExcel/Calculation.php b/Classes/PHPExcel/Calculation.php
index 5c5cb1e7..8debb323 100644
--- a/Classes/PHPExcel/Calculation.php
+++ b/Classes/PHPExcel/Calculation.php
@@ -1710,7 +1710,7 @@ class PHPExcel_Calculation {
 	 *	@return PHPExcel_Calculation
 	 */
 	public static function getInstance() {
-		if (!isset(self::$_instance) || is_null(self::$_instance)) {
+		if (!isset(self::$_instance) || (self::$_instance === NULL)) {
 			self::$_instance = new PHPExcel_Calculation();
 		}
 
@@ -1726,7 +1726,7 @@ class PHPExcel_Calculation {
 	 *	@return null
 	 */
 	public static function flushInstance() {
-		if (isset(self::$_instance) && !is_null(self::$_instance)) {
+		if (isset(self::$_instance) && (self::$_instance !== NULL)) {
 			self::$_instance->clearCalculationCache();
 		}
 	}	//	function flushInstance()
@@ -2000,7 +2000,7 @@ class PHPExcel_Calculation {
 	private static $functionReplaceToLocale		= null;
 
 	public function _translateFormulaToLocale($formula) {
-		if (is_null(self::$functionReplaceFromExcel)) {
+		if (self::$functionReplaceFromExcel === NULL) {
 			self::$functionReplaceFromExcel = array();
 			foreach(array_keys(self::$_localeFunctions) as $excelFunctionName) {
 				self::$functionReplaceFromExcel[] = '/(@?[^\w\.])'.preg_quote($excelFunctionName).'([\s]*\()/Ui';
@@ -2011,7 +2011,7 @@ class PHPExcel_Calculation {
 
 		}
 
-		if (is_null(self::$functionReplaceToLocale)) {
+		if (self::$functionReplaceToLocale === NULL) {
 			self::$functionReplaceToLocale = array();
 			foreach(array_values(self::$_localeFunctions) as $localeFunctionName) {
 				self::$functionReplaceToLocale[] = '$1'.trim($localeFunctionName).'$2';
@@ -2029,7 +2029,7 @@ class PHPExcel_Calculation {
 	private static $functionReplaceToExcel		= null;
 
 	public function _translateFormulaToEnglish($formula) {
-		if (is_null(self::$functionReplaceFromLocale)) {
+		if (self::$functionReplaceFromLocale === NULL) {
 			self::$functionReplaceFromLocale = array();
 			foreach(array_values(self::$_localeFunctions) as $localeFunctionName) {
 				self::$functionReplaceFromLocale[] = '/(@?[^\w\.])'.preg_quote($localeFunctionName).'([\s]*\()/Ui';
@@ -2039,7 +2039,7 @@ class PHPExcel_Calculation {
 			}
 		}
 
-		if (is_null(self::$functionReplaceToExcel)) {
+		if (self::$functionReplaceToExcel === NULL) {
 			self::$functionReplaceToExcel = array();
 			foreach(array_keys(self::$_localeFunctions) as $excelFunctionName) {
 				self::$functionReplaceToExcel[] = '$1'.trim($excelFunctionName).'$2';
diff --git a/Classes/PHPExcel/Cell.php b/Classes/PHPExcel/Cell.php
index 81462cea..4bcca6ca 100644
--- a/Classes/PHPExcel/Cell.php
+++ b/Classes/PHPExcel/Cell.php
@@ -316,7 +316,7 @@ class PHPExcel_Cell
 	 */
 	public function setCalculatedValue($pValue = null)
 	{
-		if (!is_null($pValue)) {
+		if ($pValue !== NULL) {
 			$this->_calculatedValue = (is_numeric($pValue)) ? (float) $pValue : $pValue;
 		}
 
diff --git a/Classes/PHPExcel/DocumentProperties.php b/Classes/PHPExcel/DocumentProperties.php
index 2893fc0c..f55eedc4 100644
--- a/Classes/PHPExcel/DocumentProperties.php
+++ b/Classes/PHPExcel/DocumentProperties.php
@@ -196,7 +196,7 @@ class PHPExcel_DocumentProperties
 	 * @return	PHPExcel_DocumentProperties
 	 */
 	public function setCreated($pValue = null) {
-		if (is_null($pValue)) {
+		if ($pValue === NULL) {
 			$pValue = time();
 		} elseif (is_string($pValue)) {
 			if (is_numeric($pValue)) {
@@ -226,7 +226,7 @@ class PHPExcel_DocumentProperties
 	 * @return	PHPExcel_DocumentProperties
 	 */
 	public function setModified($pValue = null) {
-		if (is_null($pValue)) {
+		if ($pValue === NULL) {
 			$pValue = time();
 		} elseif (is_string($pValue)) {
 			if (is_numeric($pValue)) {
@@ -439,12 +439,12 @@ class PHPExcel_DocumentProperties
 	 * @return	PHPExcel_DocumentProperties
 	 */
 	public function setCustomProperty($propertyName,$propertyValue='',$propertyType=NULL) {
-		if ((is_null($propertyType)) || (!in_array($propertyType,array(self::PROPERTY_TYPE_INTEGER,
+		if (($propertyType === NULL) || (!in_array($propertyType,array(self::PROPERTY_TYPE_INTEGER,
 																	   self::PROPERTY_TYPE_FLOAT,
 																	   self::PROPERTY_TYPE_STRING,
 																	   self::PROPERTY_TYPE_DATE,
 																	   self::PROPERTY_TYPE_BOOLEAN)))) {
-			if (is_null($propertyValue)) {
+			if ($propertyValue === NULL) {
 				$propertyType = self::PROPERTY_TYPE_STRING;
 			} elseif (is_float($propertyValue)) {
 				$propertyType = self::PROPERTY_TYPE_FLOAT;
diff --git a/Classes/PHPExcel/IOFactory.php b/Classes/PHPExcel/IOFactory.php
index 7f3c23f8..50ca0aca 100644
--- a/Classes/PHPExcel/IOFactory.php
+++ b/Classes/PHPExcel/IOFactory.php
@@ -139,7 +139,7 @@ class PHPExcel_IOFactory
 				$classFile = str_replace('{0}', $writerType, $searchLocation['path']);
 
 				$instance = new $className($phpExcel);
-				if (!is_null($instance)) {
+				if ($instance !== NULL) {
 					return $instance;
 				}
 			}
@@ -169,7 +169,7 @@ class PHPExcel_IOFactory
 				$classFile = str_replace('{0}', $readerType, $searchLocation['path']);
 
 				$instance = new $className();
-				if (!is_null($instance)) {
+				if ($instance !== NULL) {
 					return $instance;
 				}
 			}
diff --git a/Classes/PHPExcel/NamedRange.php b/Classes/PHPExcel/NamedRange.php
index 72e70983..5d4c25aa 100644
--- a/Classes/PHPExcel/NamedRange.php
+++ b/Classes/PHPExcel/NamedRange.php
@@ -82,7 +82,7 @@ class PHPExcel_NamedRange
     public function __construct($pName = null, PHPExcel_Worksheet $pWorksheet, $pRange = 'A1', $pLocalOnly = false, $pScope = null)
     {
     	// Validate data
-    	if (is_null($pName) || is_null($pWorksheet)|| is_null($pRange)) {
+    	if (($pName === NULL) || ($pWorksheet === NULL) || ($pRange === NULL)) {
     		throw new Exception('Parameters can not be null.');
     	}
 
@@ -111,17 +111,17 @@ class PHPExcel_NamedRange
      * @return PHPExcel_NamedRange
      */
     public function setName($value = null) {
-    	if (!is_null($value)) {
+    	if ($value !== NULL) {
     		// Old title
     		$oldTitle = $this->_name;
 
     		// Re-attach
-    		if (!is_null($this->_worksheet)) {
+    		if ($this->_worksheet !== NULL) {
     			$this->_worksheet->getParent()->removeNamedRange($this->_name,$this->_worksheet);
     		}
     		$this->_name = $value;
 
-    		if (!is_null($this->_worksheet)) {
+    		if ($this->_worksheet !== NULL) {
     			$this->_worksheet->getParent()->addNamedRange($this);
     		}
 
@@ -148,7 +148,7 @@ class PHPExcel_NamedRange
      * @return PHPExcel_NamedRange
      */
     public function setWorksheet(PHPExcel_Worksheet $value = null) {
-    	if (!is_null($value)) {
+    	if ($value !== NULL) {
     		$this->_worksheet = $value;
     	}
     	return $this;
@@ -170,7 +170,7 @@ class PHPExcel_NamedRange
      * @return PHPExcel_NamedRange
      */
     public function setRange($value = null) {
-    	if (!is_null($value)) {
+    	if ($value !== NULL) {
     		$this->_range = $value;
     	}
     	return $this;
diff --git a/Classes/PHPExcel/Reader/OOCalc.php b/Classes/PHPExcel/Reader/OOCalc.php
index 0a17d157..05657721 100644
--- a/Classes/PHPExcel/Reader/OOCalc.php
+++ b/Classes/PHPExcel/Reader/OOCalc.php
@@ -500,7 +500,7 @@ class PHPExcel_Reader_OOCalc implements PHPExcel_Reader_IReader
 													break;
 										}
 //										echo 'Data value is '.$dataValue.'<br />';
-//										if (!is_null($hyperlink)) {
+//										if ($hyperlink !== NULL) {
 //											echo 'Hyperlink is '.$hyperlink.'<br />';
 //										}
 									}
@@ -525,7 +525,7 @@ class PHPExcel_Reader_OOCalc implements PHPExcel_Reader_IReader
 //										echo 'Adjusted Formula: '.$cellDataFormula.'<br />';
 									}
 
-									if (!is_null($type)) {
+									if ($type !== NULL) {
 										$objPHPExcel->getActiveSheet()->getCell($columnID.$rowID)->setValueExplicit((($hasCalculatedValue) ? $cellDataFormula : $dataValue),$type);
 										if ($hasCalculatedValue) {
 //											echo 'Forumla result is '.$dataValue.'<br />';
@@ -535,7 +535,7 @@ class PHPExcel_Reader_OOCalc implements PHPExcel_Reader_IReader
 											($cellDataOfficeAttributes['value-type'] == 'time')) {
 											$objPHPExcel->getActiveSheet()->getStyle($columnID.$rowID)->getNumberFormat()->setFormatCode($formatting);
 										}
-										if (!is_null($hyperlink)) {
+										if ($hyperlink !== NULL) {
 											$objPHPExcel->getActiveSheet()->getCell($columnID.$rowID)->getHyperlink()->setUrl($hyperlink);
 										}
 									}
diff --git a/Classes/PHPExcel/RichText.php b/Classes/PHPExcel/RichText.php
index 2391fcb3..77890269 100644
--- a/Classes/PHPExcel/RichText.php
+++ b/Classes/PHPExcel/RichText.php
@@ -54,7 +54,7 @@ class PHPExcel_RichText implements PHPExcel_IComparable
     	$this->_richTextElements = array();
 
     	// Rich-Text string attached to cell?
-    	if (!is_null($pCell)) {
+    	if ($pCell !== NULL) {
 	    	// Add cell text and style
 	    	if ($pCell->getValue() != "") {
 	    		$objRun = new PHPExcel_RichText_Run($pCell->getValue());
diff --git a/Classes/PHPExcel/Shared/File.php b/Classes/PHPExcel/Shared/File.php
index 24ba66a0..31d10552 100644
--- a/Classes/PHPExcel/Shared/File.php
+++ b/Classes/PHPExcel/Shared/File.php
@@ -80,7 +80,7 @@ class PHPExcel_Shared_File
 		}
 
 		// Found something?
-		if ($returnValue == '' || is_null($returnValue)) {
+		if ($returnValue == '' || ($returnValue === NULL)) {
 			$pathArray = explode('/' , $pFilename);
 			while(in_array('..', $pathArray) && $pathArray[0] != '..') {
 				for ($i = 0; $i < count($pathArray); ++$i) {
diff --git a/Classes/PHPExcel/Shared/OLE/PPS/Root.php b/Classes/PHPExcel/Shared/OLE/PPS/Root.php
index 532e652f..2c1a4a40 100644
--- a/Classes/PHPExcel/Shared/OLE/PPS/Root.php
+++ b/Classes/PHPExcel/Shared/OLE/PPS/Root.php
@@ -79,7 +79,7 @@ class PHPExcel_Shared_OLE_PPS_Root extends PHPExcel_Shared_OLE_PPS
 		if (is_resource($filename)) {
 		    $this->_FILEH_ = $filename;
 		} else if ($filename == '-' || $filename == '') {
-			if (is_null($this->_tmp_dir))
+			if ($this->_tmp_dir === NULL)
 				$this->_tmp_dir = PHPExcel_Shared_File::sys_get_temp_dir();
 			$this->_tmp_filename = tempnam($this->_tmp_dir, "OLE_PPS_Root");
 			$this->_FILEH_ = fopen($this->_tmp_filename,"w+b");
diff --git a/Classes/PHPExcel/Shared/OLERead.php b/Classes/PHPExcel/Shared/OLERead.php
index f4e0d69c..c2d45e90 100644
--- a/Classes/PHPExcel/Shared/OLERead.php
+++ b/Classes/PHPExcel/Shared/OLERead.php
@@ -175,7 +175,7 @@ class PHPExcel_Shared_OLERead {
 	 */
 	public function getStream($stream)
 	{
-		if (is_null($stream)) {
+		if ($stream === NULL) {
 			return null;
 		}
 
diff --git a/Classes/PHPExcel/Shared/XMLWriter.php b/Classes/PHPExcel/Shared/XMLWriter.php
index c7dfeb56..2a8ddec5 100644
--- a/Classes/PHPExcel/Shared/XMLWriter.php
+++ b/Classes/PHPExcel/Shared/XMLWriter.php
@@ -65,7 +65,7 @@ class PHPExcel_Shared_XMLWriter extends XMLWriter {
 			$this->openMemory();
 		} else {
 			// Create temporary filename
-			if (is_null($pTemporaryStorageFolder))
+			if ($pTemporaryStorageFolder === NULL)
 				$pTemporaryStorageFolder = PHPExcel_Shared_File::sys_get_temp_dir();
 			$this->_tempFileName = @tempnam($pTemporaryStorageFolder, 'xml');
 
diff --git a/Classes/PHPExcel/Worksheet.php b/Classes/PHPExcel/Worksheet.php
index c4c1e652..d805626c 100644
--- a/Classes/PHPExcel/Worksheet.php
+++ b/Classes/PHPExcel/Worksheet.php
@@ -1076,7 +1076,7 @@ class PHPExcel_Worksheet implements PHPExcel_IComparable
 		if ((!preg_match('/^'.PHPExcel_Calculation::CALCULATION_REGEXP_CELLREF.'$/i', $pCoordinate, $matches)) &&
 			(preg_match('/^'.PHPExcel_Calculation::CALCULATION_REGEXP_NAMEDRANGE.'$/i', $pCoordinate, $matches))) {
 			$namedRange = PHPExcel_NamedRange::resolveRange($pCoordinate, $this);
-			if (!is_null($namedRange)) {
+			if ($namedRange !== NULL) {
 				$pCoordinate = $namedRange->getRange();
 				if ($this->getHashCode() != $namedRange->getWorksheet()->getHashCode()) {
 					if (!$namedRange->getLocalOnly()) {
@@ -2278,7 +2278,7 @@ class PHPExcel_Worksheet implements PHPExcel_IComparable
 	 */
 	public function namedRangeToArray($pNamedRange = '', $nullValue = null, $calculateFormulas = true, $formatData = true, $returnCellRef = false) {
 		$namedRange = PHPExcel_NamedRange::resolveRange($pNamedRange, $this);
-		if (!is_null($namedRange)) {
+		if ($namedRange !== NULL) {
 			$pWorkSheet = $namedRange->getWorksheet();
 			$pCellRange = $namedRange->getRange();
 
@@ -2551,7 +2551,7 @@ class PHPExcel_Worksheet implements PHPExcel_IComparable
 	 */
 	public function getTabColor()
 	{
-		if (is_null($this->_tabColor))
+		if ($this->_tabColor === NULL)
 			$this->_tabColor = new PHPExcel_Style_Color();
 
 		return $this->_tabColor;
@@ -2577,7 +2577,7 @@ class PHPExcel_Worksheet implements PHPExcel_IComparable
 	 */
 	public function isTabColorSet()
 	{
-		return !is_null($this->_tabColor);
+		return ($this->_tabColor !== NULL);
 	}
 
 	/**
diff --git a/Classes/PHPExcel/Writer/Excel2007/StringTable.php b/Classes/PHPExcel/Writer/Excel2007/StringTable.php
index 8165df1f..3e429cda 100644
--- a/Classes/PHPExcel/Writer/Excel2007/StringTable.php
+++ b/Classes/PHPExcel/Writer/Excel2007/StringTable.php
@@ -45,7 +45,7 @@ class PHPExcel_Writer_Excel2007_StringTable extends PHPExcel_Writer_Excel2007_Wr
 	 */
 	public function createStringTable($pSheet = null, $pExistingTable = null)
 	{
-		if (!is_null($pSheet)) {
+		if ($pSheet !== NULL) {
 			// Create string lookup table
 			$aStringTable = array();
 			$cellCollection = null;
@@ -64,14 +64,14 @@ class PHPExcel_Writer_Excel2007_StringTable extends PHPExcel_Writer_Excel2007_Wr
 				$cell = $pSheet->getCell($cellID);
 				$cellValue = $cell->getValue();
 				if (!is_object($cellValue) &&
-					!is_null($cellValue) &&
+					($cellValue !== NULL) &&
 					$cellValue !== '' &&
 					!isset($aFlippedStringTable[$cellValue]) &&
 					($cell->getDataType() == PHPExcel_Cell_DataType::TYPE_STRING || $cell->getDataType() == PHPExcel_Cell_DataType::TYPE_STRING2 || $cell->getDataType() == PHPExcel_Cell_DataType::TYPE_NULL)) {
 						$aStringTable[] = $cellValue;
 						$aFlippedStringTable[$cellValue] = true;
 				} elseif ($cellValue instanceof PHPExcel_RichText &&
-						  !is_null($cellValue) &&
+						  ($cellValue !== NULL) &&
 						  !isset($aFlippedStringTable[$cellValue->getHashCode()])) {
 								$aStringTable[] = $cellValue;
 								$aFlippedStringTable[$cellValue->getHashCode()] = true;