I found the reason thanks to checking the UNIX_TIMESTAMP() call in a MySQL server of the same system, as we've noticed that this was a completely outdated version too (5.5.6). Turns out that both the UNIX_TIMESTAMP method in such old MySQL versions as well as in PHP 5.6 only properly compute timestamps until the year 2037. The UNIX_TIMESTAMP method simply fails for years afterwards by returning 0, and the PHP methods return an incorrect timestamp.