How to make strpos() only match whole numbers in a delimited string?

I have a comma delimited string and I need to be able to search the string for instances of a given string. I use the following function:

function isChecked($haystack, $needle) {
    $pos = strpos($haystack, $needle);
    if ($pos === false) {
        return null;
    } else {
        'return 'checked="checked"';
    }
}

Example: isChecked('1,2,3,4', '2') searches if 2 is in the string and ticks the appropriate checkbox in one of my forms.

When it comes to isChecked('1,3,4,12', '2') though, instead of returning NULL it returns TRUE, as it obviously finds the character 2 within 12.

How should I use the strpos function in order to have only the correct results?

function isChecked($haystack, $needle) {
    $haystack = explode(',', $haystack);
    return in_array($needle, $haystack);
}

Also you can use regular expressions