PHP replace string (only replace the first string)

$str=preg_replace('/abc/','xyz',$str,1);
echo $str;

Do not use regular string replacement code:

$replace='xyz';
if(($position=strpos($str,$replace))!==false){
 $leng=strlen($replace);
 $str=substr_replace ($str,'xyz',$position,$leng);
}
echo $str;