$str_time = "23:12:95";

$str_time = preg_replace("/^([\d]{1,2})\:([\d]{2})$/", "00:$1:$2", $str_time);

sscanf($str_time, "%d:%d:%d", $hours, $minutes, $seconds);

$time_seconds = $hours * 3600 + $minutes * 60 + $seconds;

$str_time = "2:50";

sscanf($str_time, "%d:%d:%d", $hours, $minutes, $seconds);

$time_seconds = isset($seconds) ? $hours * 3600 + $minutes * 60 + $seconds : $hours * 60 + $minutes;