Strstr finds first occurrence of a string inside another string,stristr() is idential to strstr() except that it is case insensitive.
strstr() returns part of a given string from the first occurrence of a given substring to the end of the string. For example: strstr("user@example.com","@") will return "@example.com".
stristr() is idential to strstr() except that it is case insensitive.
