Convert time in HH:MM:SS format to seconds only?

No need to explode anything:

$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;

And if you don't want to use regular expressions:

$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;

I think the easiest method would be to use strtotime() function:

$time = '21:30:10';
$seconds = strtotime("1970-01-01 $time UTC");
echo $seconds;

// same with objects (for php5.3+)
$time = '21:30:10';
$dt = new DateTime("1970-01-01 $time", new DateTimeZone('UTC'));
$seconds = (int)$dt->getTimestamp();
echo $seconds;

demo

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Function date_parse() can also be used for parsing date and time:

$time = '21:30:10';
$parsed = date_parse($time);
$seconds = $parsed['hour'] * 3600 + $parsed['minute'] * 60 + $parsed['second'];

demo

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If you will parse format MM:SS with strtotime() or date_parse() it will fail (date_parse() is used in strtotime() and DateTime), because when you input format like xx:yy parser assumes it is HH:MM and not MM:SS. I would suggest checking format, and prepend 00: if you only have MM:SS.

demo strtotime() demo date_parse()

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If you have hours more than 24, then you can use next function (it will work for MM:SS and HH:MM:SS format):

function TimeToSec($time) {
    $sec = 0;
    foreach (array_reverse(explode(':', $time)) as $k => $v) $sec += pow(60, $k) * $v;
    return $sec;
}

demo

    $time = 00:06:00;
    $timeInSeconds = strtotime($time) - strtotime('TODAY');