strstr() function is used to return the sub string from first occurrence of string point from the string base.If string point is not found, returns FALSE. stristr() function is also but it is case insensitive.

strstr() example

<?php
$username= 'deepak';
$substring_username = strstr($username, 'p');
echo $substring_username; // prints pak
?>

stristr() example

<?php
$username= 'DEEPAK';
$substring_username = stristr($username, 'p');
echo $substring_username;
// outputs PAK.

Thanks