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# PHP preg_replace_callback() Function [PHP Regular Expressions (PCRE)](#) The preg_replace_callback function performs a regular expression search and uses a callback for replacement. ### Syntax mixed preg_replace_callback ( mixed $pattern , callable $callback , mixed $subject [, int $limit = -1 [, int &$count ]] ) This function behaves similarly to preg_replace(), except that instead of a replacement string, a callback is specified for the replacement calculation. Parameter Description: * $pattern: The pattern to search for. Can be a string or an array of strings. * $callback: A callback function that is called for each match. The function receives the matched results from the subject as parameters. * $subject: The target string or array of strings to search and replace. * $limit: Optional. The maximum number of replacements for each pattern in each subject string. Default is -1 (no limit). * $count: Optional. The number of replacements performed. ### Return Value If subject is an array, preg_replace_callback() returns an array; otherwise, it returns a string. Returns NULL on error. If a match is found, the replaced target string (or array of strings) is returned; otherwise, the subject is returned unchanged. ### Example ## Example 1 The execution result is as follows: April fools day is 04/01/2003Last christmas was 12/24/2002 [PHP Regular Expressions (PCRE)](#)