- str_replace: documentation ( source)
- trim: documentation ( source)
<?php
$main_string = "23/CSE/4/2014";
$sub_string = "CSE/4/2014";
echo trim(str_replace($sub_string, '', $main_string), '/');
echo "\n";
$main_string = "CSE/4/2014/23";
$sub_string = "CSE/4/2014";
echo trim(str_replace($sub_string, '', $main_string), '/');