You want the number of days until Friday, in other words.
Timestamp for that would be calculated by
$this_weekday=date('w');
$interval=(5-$this_weekday)*86400); // Seconds until Friday
Add $interval to the current time (time()), and you have a timestamp for the appropriate Friday.
Now, do you want the week of Friday 8 November to end on Friday 8 November? In other words, does the week end when the Friday ends or when it starts? If the latter, then put
if ($this_weekday==5) $this_weekday++; // Today's Friday - we want next Friday.
before you calculate the $interval.