Two versions, equal in performance according to simple microtime testing. http://stackoverflow.com/questions/1252693/php-str-replace-that-only-acts-on-the-first-match

str_replace_first.php

<?php
/**
* Replace the first occurrence in a string.
*
* @param string $search The value being searched for.
* @param string $replace The replacement value.
* @param string $subject The string being searched.
* @return string
*/
function str_replace_first( $search, $replace, $subject ) {
	return implode( $replace, explode( $search, $subject, 2 ) );
}

substr_replace-version.php

<?php
/**
 * Replace the first occurrence in a string.
 *
 * @param string $search The value being searched for.
 * @param string $replace The replacement value.
 * @param string $subject The string being searched.
 * @return string
 */
function str_replace_first( $search, $replace, $subject ) {
	$pos = strpos( $subject, $search );

	if ( false !== $pos )
		$subject = substr_replace( $subject, $replace, $pos, strlen( $search ) );

	return $subject;
}