diff --git a/library/HTMLPurifier/Lexer/DirectLex.php b/library/HTMLPurifier/Lexer/DirectLex.php
index fa9d541a..e5643758 100644
--- a/library/HTMLPurifier/Lexer/DirectLex.php
+++ b/library/HTMLPurifier/Lexer/DirectLex.php
@@ -71,7 +71,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                 $cursor > 0 &&            // cursor is further than zero
                 $loops % $synchronize_interval === 0 // time to synchronize!
             ) {
-                $current_line = 1 + substr_count($html, $nl, 0, $cursor);
+                $current_line = 1 + $this->substrCount($html, $nl, 0, $cursor);
             }
             
             $position_next_lt = strpos($html, '<', $cursor);
@@ -96,7 +96,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                     );
                 if ($maintain_line_numbers) {
                     $token->line = $current_line;
-                    $current_line += substr_count($html, $nl, $cursor, $position_next_lt - $cursor);
+                    $current_line += $this->substrCount($html, $nl, $cursor, $position_next_lt - $cursor);
                 }
                 $array[] = $token;
                 $cursor  = $position_next_lt + 1;
@@ -137,7 +137,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                         );
                     if ($maintain_line_numbers) {
                         $token->line = $current_line;
-                        $current_line += substr_count($html, $nl, $cursor, $position_next_gt - $cursor);
+                        $current_line += $this->substrCount($html, $nl, $cursor, $position_next_gt - $cursor);
                     }
                     $array[] = $token;
                     $inside_tag = false;
@@ -152,7 +152,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                     $token = new HTMLPurifier_Token_End($type);
                     if ($maintain_line_numbers) {
                         $token->line = $current_line;
-                        $current_line += substr_count($html, $nl, $cursor, $position_next_gt - $cursor);
+                        $current_line += $this->substrCount($html, $nl, $cursor, $position_next_gt - $cursor);
                     }
                     $array[] = $token;
                     $inside_tag = false;
@@ -174,7 +174,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                         );
                     if ($maintain_line_numbers) {
                         $token->line = $current_line;
-                        $current_line += substr_count($html, $nl, $cursor, $position_next_gt - $cursor);
+                        $current_line += $this->substrCount($html, $nl, $cursor, $position_next_gt - $cursor);
                     }
                     $array[] = $token;
                     $cursor = $position_next_gt + 1;
@@ -203,7 +203,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                     }
                     if ($maintain_line_numbers) {
                         $token->line = $current_line;
-                        $current_line += substr_count($html, $nl, $cursor, $position_next_gt - $cursor);
+                        $current_line += $this->substrCount($html, $nl, $cursor, $position_next_gt - $cursor);
                     }
                     $array[] = $token;
                     $inside_tag = false;
@@ -235,7 +235,7 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
                 }
                 if ($maintain_line_numbers) {
                     $token->line = $current_line;
-                    $current_line += substr_count($html, $nl, $cursor, $position_next_gt - $cursor);
+                    $current_line += $this->substrCount($html, $nl, $cursor, $position_next_gt - $cursor);
                 }
                 $array[] = $token;
                 $cursor = $position_next_gt + 1;
@@ -259,6 +259,22 @@ class HTMLPurifier_Lexer_DirectLex extends HTMLPurifier_Lexer
         return $array;
     }
     
+    /**
+     * PHP 4 compatible substr_count that implements offset and length
+     */
+    function substrCount($haystack, $needle, $offset, $length) {
+        static $oldVersion;
+        if ($oldVersion === null) {
+            $oldVersion = version_compare(PHP_VERSION, '5.1', '<');
+        }
+        if ($oldVersion) {
+            $haystack = substr($haystack, $offset, $length);
+            return substr_count($haystack, $needle);
+        } else {
+            return substr_count($haystack, $needle, $offset, $length);
+        }
+    }
+    
     /**
      * Takes the inside of an HTML tag and makes an assoc array of attributes.
      *