What is the functionality of the function strstr and stristr? | PHP

strstr Returns part of string from the first occurrence of needle(sub string that we finding out ) to the end of string. 

$email= 'sonialouder@gmail.com';

$domain = strstr($email, '@');

echo $domain; // prints @gmail.com

here @ is the needle 

stristr is case-insensitive means able not able to diffrenciate between a and A