Date and time in 24 hours format

https://www.w3schools.com/php/php_date.asp

Get a Date

The required format parameter of the date() function specifies how to format the date (or time).

Here are some characters that are commonly used for dates:

• d - Represents the day of the month (01 to 31)
• m - Represents a month (01 to 12)
• Y - Represents a year (in four digits)
• l (lowercase 'L') - Represents the day of the week

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Get a Time

Here are some characters that are commonly used for times:

• H - 24-hour format of an hour (00 to 23)
• h - 12-hour format of an hour with leading zeros (01 to 12)
• i - Minutes with leading zeros (00 to 59)
• s - Seconds with leading zeros (00 to 59)
• a - Lowercase Ante meridiem and Post meridiem (am or pm)

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https://www.php.net/manual/en/datetime.format.php

public DateTime::format ( string $format ) : string

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<?php

    $datetime = new DateTime( "now", new DateTimeZone( "Europe/Bucharest" ) );

    echo $datetime->format( 'Y-m-d H:i:s' );

Use H instead:

$test = 'Fri, 15 Jan 2016 15:14:10 +0800';
$t = date('Y-m-d H:i:s',strtotime($test));
echo $t;

H: 24-hour format of an hour with leading zeros 00 through 23

G should be the same, but without leading zeroes though. I suspect that your PHP is set to a different timezone than +0800. Can you confirm your timezone (date_default_timezone_get())?

EDIT

OP confirmed that his timezone was set to UTC, in which case it maskes perfect sense that it shows 7 in the morning, as date uses PHPs default timezone.

If you want to "inherit" the Timezone, while getting more flexibility, you should switch to DateTime:

echo (new DateTime($test))->format("Y-m-d H:i:s");