Converting hours, minutes and seconds (H:i:s) string to just seconds with PHP can be done with an adaptable function that also works for just a minutes and seconds string.

The time string to seconds function:

function timeToSeconds(string $time): int
{
    $arr = explode(':', $time);
    if (count($arr) === 3) {
        return $arr[0] * 3600 + $arr[1] * 60 + $arr[2];
    }
    return $arr[0] * 60 + $arr[1];
}

Example calls:

echo timeToSeconds('6:10');//370

echo timeToSeconds('1:6:14');//3974

The function works by checking if the string after being split, contains 3 elements which are hours, minutes and seconds otherwise it is just minutes and seconds.

Then the correct elements get multiplied by their respective seconds, i.e an hour is 3600 seconds and a minute is 60.