When I check to see online here the Weekday of 622-07-19 it shows Friday. However when I use PHP it shows shows Saturday. Which one is correctly showing the Weekday?
<?php echo date('Y-m-d H:i:s l', strtotime('0622-07-19 00:00:00'));?>
If you're talking about the Gregorian calendar, your online checker is correct - it's Friday, and I don't understand the PHP output. It may be that it's interpreting it as a Julian date, but that would also be incorrect.
I suspect you do mean the Gregorian calendar, as July 19th 622 (Gregorian) is the civil epoch for the Islamic calendar. This is equal to July 16th 622 (Julian) which is the more common way of referring to the date.
From the Wikipedia article on the Islamic Calendar:
The first day of the first month of the Islamic calendar (1 Muharram 1 AH) was Friday, 16 July AD 622, the equivalent civil tabular date (same daylight period) in the Julian calendar.[22][23] The Islamic day began at the preceding sunset on the evening of 15 July.
(The part about days starting at sunset confuses things a lot, but I suggest you ignore that for the moment - it wouldn't explain your PHP result.)
If you're interested in finding out more about where PHP is going wrong, I suggest you do a binary chop to find out exactly where the problem is - try various dates in both PHP and other sources. So try today (which is highly likely to be correct, but it's worth it just for sanity) then half way between 622 and today, etc.
Once you've worked out a date at which PHP becomes correct, it should be easier to work out why that's happening.