I know this topic is old, but this is a shorter way to do it with elvis operator and "+7 day" expression for strtotime():
$week=date("W",strtotime(date("w")==1?"+7 day":"+0 day"));
if $date("w") returns true means today is a day between tuesday and sunday (1-6), so, we can return today week ('today').
if returns false, it means is monday (0), so, we should return the next day ('+1 week').
This way we don't need to care about last or first day of year or check if current year has 52 or 53 weeks.
Edited: the previous answer (and others in this topic) doesn't work for this year because januray 1st is monday, so, it needs to be 1 week ago (-1 week) excluding sunday (day 6).
date("W",strtotime(date("w")?'-7 day':'+0 day'));
I think a condition asking if januray 1st is monday could work, but I didn't test it yet, I will come back with an answer later
For a custom day you could use this:
$date = strtotime('2018-04-30'); // (it is monday)
if(date("w",strtotime(date('Y',$date).'-01-01'))==1){ // if first day of year is monday
$week = strtotime(date('w',$date)?"-7 day":"+0 day",$date); // and today is sunday, sub a week
$week = date("W",$week);
}else{ // if is not monday
$week = strtotime(date('w',$date)==1?"+7 day":"+0 day",$date); // and today is monday, add a week
$week = date("W",$week);
}