preg_replace_callback Perform a regular expression search and replace using a callback mixedpreg_replace_callback mixedpattern callbackcallback mixedsubject intlimit The behavior of this function is almost identical to preg_replace, except for the fact that instead of replacement parameter, one should specify a callback that will be called and passed an array of matched elements in the subject string. The callback should return the replacement string. ]]> You'll often need the callback function for a preg_replace_callback in just one place. In this case you can use create_function to declare an anonymous function as callback within the call to preg_replace_callback. By doing it this way you have all information for the call in one place and do not clutter the function namespace with a callback functions name not used anywhere else. \s*\w|', create_function( // single quotes are essential here, // or alternative escape all $ as \$ '$matches', 'return strtolower($matches[0]);' ), $line ); echo $line; } fclose($fp); ?> ]]> See also preg_replace, create_function.