str_replace_first

function str_replace_first($needle, $replacement, $haystack) {
	$needle_start = strpos($haystack, $needle);
	$needle_end = $needle_start + strlen($needle);
	if($needle_start!==false) {
		$to_replace = substr($haystack, 0, $needle_end);
		return str_replace($needle, $replacement, $to_replace) . substr($haystack, $needle_end);
	}
	else
		return $haystack;
}