Yep, I did that and it works. Though it doesn't reset always on the same time, it will reset at least weekly, which should work.
$time_file = 'timer.file';
$ttime = file_get_contents( $time_file );
if( $ttime < time() )
{
mysql_query( "UPDATE topp5_trailer SET week_sott = 0" );
$fp = fopen( $time_file, 'w+' );
fwrite( $fp, time() + 604800 );
fclose( $fp );
}