php replace first occurrence of string from 0th position -

i want search , replace first word in php follows:

$str="nothing inside"; 

replace 'nothing' 'something' search , replace without using substr

output should be: 'something inside'

use preg_replace() limit of 1:

preg_replace('/nothing/', 'something', $str, 1); 

replace regular expression /nothing/ whatever string want search for. since regular expressions evaluated left-to-right, match first instance.