不需要explode任何东西：

$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;

而且，如果您不想使用正则表达式：

$str_time = "2:50";

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

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