'first wed Jan 2020' vs 'first wed of Jan 2020' in php-7

Question

Today I encountered a strange bug/feature in PhP-7 reported by a colleague in an application I wrote. It boils down to the following.

Consider the following:

echo date('Y-m-d', strtotime('first thu of Jan 2020'));
echo '<br/>';
echo date('Y-m-d', strtotime('first thu Jan 2020'));
echo '<br/>';
echo date('Y-m-d', strtotime('first wed of Jan 2020'));
echo '<br/>';
echo date('Y-m-d', strtotime('first wed Jan 2020'));

When I run it, I see the following:

2020-01-02
2020-01-02
2020-01-01
2020-01-08

Strangely 3rd and 4th lines of the output are different. Why? Is it a bug or one must use of in such cases?

Answer 1

This behaviour is explicity mentioned in documentation (Second Note group, row 4):

"ordinal dayname" does advance to another day. (Example "first wednesday july 23rd, 2008" means "2008-07-30").

And see also the first Note:

Relative statements are always processed after non-relative statements. This makes "+1 week july 2008" and "july 2008 +1 week" equivalent.

So if you don't write the word "of" the processor does not catch the statement "first day of". Consequently it split the string in two statements: first of all set the date to 1 Jan 2020 (through the statement Jan 2020 that it's interpreted as non-relative statement) then it applies the relative statement first wed as mentioned above.

Answer 2

According to the documentation the format you have to use is

ordinal space dayname space 'of'

So the 'of' is not optional. I cannot tell, why it is working in some cases without, but it looks like a coincidence.