php strtotime 'third day of + 1 month' = false

Question

I want a loop like this:

This loop is OK:

while($d<=$d2) {  
        $x = "first day of + 1 month";
        $d=strtotime($x,$d);
        do_something($d);
}

But

while($d<=$d2) {  
        $x = "third day of + 1 month";
        $d=strtotime($x,$d);
        do_something($d);
}

doens't work: https://www.functions-online.com/strtotime.html gives "false" How can I solve this?

(first, second, third etc is a variable)

Answer 1

Without specifying a day of the week, only the first and last day work with "+ x months". It only remains to work twice with strtotime. Example:

$d1= strtotime('first day of + 1 month',$d);
$day = 3;
$dateStr = date("Y-m-",$d1).sprintf("%02d",$day);
$d = strtotime($dateStr);