Check if string has a specific number, not digit

This is what I have:

$a = '3,24,57';

if (strpos($a, '7') == true) {
    echo 'number found';
}

The code will return "number found" because of the number 57 but there is no number 7 in the string. How can I make this work in a way that will return true only if the string is like this: "3,7,24,57"

Thanks

>Solution :

Just try like this.

$array = explode(",", $a);
if (in_array("7", $array )) {
    echo 'number found';
}