PHP strpos() function comparision of string conflicts in numbers

I am using strpos() function to find the string in an array key members.

    foreach($_POST as $k => $v){
    // for all types of questions of chapter 1 only
    $count = 0;
    if(strpos($k, 'chap1') !== false){
        $count++;
    }
}

I know that it works only until the keys are (chap1e1, chap1m1, chap1h1) but when it comes to (chap10e1, chap10m1, chap10h1), my logic won't be working on those.

Isn't there any way, so that, I can distinguish the comparison between (chap1 & chap10)?

Or, Is there any alternative way of doing this? Please give me some ideas on it. Thank you!