If we want to know if a string contains a particular substring. For example, you want to find out if an email address contains a @

Use strpos( )

Finding a substring with strpos( )

<?php
if (strpos($_POST['email'], '@') === false) {
   print 'There was no @ in the e-mail address!';
 }
?>

The return value from strpos( ) is the first position in the string
(the "haystack") at which the substring (the "needle") was found.
If the needle wasn't found at all in the haystack, strpos( ) returns
false. If the needle is at the beginning of the haystack, strpos( )
returns 0, since position 0 represents the beginning of the string.
To differentiate between return values of 0 and false, you must
use the identity operator (===) or the notidentity operator (!==)
instead of regular equals (==) or not-equals (!=).