Adding to date in a while loop - php

Question

I have a date that goes into a loop that the user specifies. The date will always come from the database formatted as a 'Y-m-d' string. I am aware that I can compare the strings directly as long as they are in that format, however, I have also tried using strtotime to convert the dates to compare them with no luck. I am trying to determine how many paycheck a user has before a payment is due
Here is what I have
$due_date = '2016-12-13';
//count paychecks set to zero and added to by loop
$paychecks = 0;
//users next paycheck ('Y-m-d' ALWAYS)
$next_payday = $user['next_payday']; //equal to '2016-12-02'
//how often they get paid (int)
$frequency = 14;
while(strtotime($next_payday) <= strtotime($due_date)){
//next_payday equals 1480654800 when coming into the loop
//due_date equals 1481605200 when coming into the loop
//add 14 days to the date
$next_payday = date('Y-m-d', strtotime("+" .$frequency." days"));;
//add to paychecks
$paychecks++;
}
The problem is that the loop never stops. It keeps going and going.
Thanks for any help anyone can give me.

Answer 1

Ah, be sure to use strtotime to get integers (representing number of seconds since the epoch) for comparison and multiply your frequency of days by the number of seconds in a day (86400):
$due_date = strtotime('2016-12-25');
//count paychecks set to zero and added to by loop
$paychecks = 0;
//users next paycheck (unixtime for comparison)
$next_payday = strtotime($user['next_payday']);
//how often they get paid (int)
$frequency = 14;
while($next_payday <= $due_date){
//add 14 days to the date
$next_payday += ($frequency * 86400);
//add to paychecks
$paychecks++;
}