directly from PHP. See credits at bottom. Back to my summary.
The following characters are recognized in the format parameter string
format character Description Example returned values
Day --- ---
d Day of the month, 2 digits with leading zeros 01 to 31
D A textual representation of a day, three letters Mon through Sun
j Day of the month without leading zeros 1 to 31
l (lowercase 'L') A full textual representation of the day of the week Sunday through Saturday
N ISO-8601 numeric representation of the day of the week (added in PHP 5.1.0) 1 (for Monday) through 7 (for Sunday)
S English ordinal suffix for the day of the month, 2 characters st, nd, rd or th. Works well with j
w Numeric representation of the day of the week 0 (for Sunday) through 6 (for Saturday)
z The day of the year (starting from 0) 0 through 365
Week --- ---
W ISO-8601 week number of year, weeks starting on Monday Example: 42 (the 42nd week in the year)
Month --- ---
F A full textual representation of a month, such as January or March January through December
m Numeric representation of a month, with leading zeros 01 through 12
M A short textual representation of a month, three letters Jan through Dec
n Numeric representation of a month, without leading zeros 1 through 12
t Number of days in the given month 28 through 31
Year --- ---
L Whether it's a leap year 1 if it is a leap year, 0 otherwise.
o ISO-8601 week-numbering year. This has the same value as Y, except that if the ISO week number (W) belongs to the previous or next year, that year is used instead. (added in PHP 5.1.0) Examples: 1999 or 2003
Y A full numeric representation of a year, 4 digits Examples: 1999 or 2003
y A two digit representation of a year Examples: 99 or 03
Time --- ---
a Lowercase Ante meridiem and Post meridiem am or pm
A Uppercase Ante meridiem and Post meridiem AM or PM
B Swatch Internet time 000 through 999
g 12-hour format of an hour without leading zeros 1 through 12
G 24-hour format of an hour without leading zeros 0 through 23
h 12-hour format of an hour with leading zeros 01 through 12
H 24-hour format of an hour with leading zeros 00 through 23
i Minutes with leading zeros 00 to 59
s Seconds with leading zeros 00 through 59
u Microseconds (added in PHP 5.2.2). Note that date() will always generate 000000 since it takes an integer parameter, whereas DateTime::format() does support microseconds if DateTime was created with microseconds. Example: 654321
v Milliseconds (added in PHP 7.0.0). Same note applies as for u. Example: 654
Timezone --- ---
e Timezone identifier (added in PHP 5.1.0) Examples: UTC, GMT, Atlantic/Azores
I (capital i) Whether or not the date is in daylight saving time 1 if Daylight Saving Time, 0 otherwise.
O Difference to Greenwich time (GMT) without colon between hours and minutes Example: +0200
P Difference to Greenwich time (GMT) with colon between hours and minutes (added in PHP 5.1.3) Example: +02:00
T Timezone abbreviation Examples: EST, MDT ...
Z Timezone offset in seconds. The offset for timezones west of UTC is always negative, and for those east of UTC is always positive. -43200 through 50400
Full Date/Time --- ---
c ISO 8601 date (added in PHP 5) 2004-02-12T15:19:21+00:00
r » RFC 2822 formatted date Example: Thu, 21 Dec 2000 16:01:07 +0200
U Seconds since the Unix Epoch (January 1 1970 00:00:00 GMT) See also time()

Credits

This table is from the above-mentioned page. I have not changed the content other than I had to add absolute URLs. I did change the external css. I mercilessly chopped the Bootstrap and other CSS until I had just what I want. So the table looks close to the original but I have made changes. I copied the table 2020/07/31 at around 10pm EDT. At this time, PHP 7.4 is current and has been so for months. I am running v7.4.8 which should be within days of current through Ubuntu and Ondrej's package source. PHP 8.x is starting to be discussed, within the last few weeks; specifically, "PHP 8.0.0 Alpha 3 available for testing."

The exact page, by reference, is

(PHP 5 >= 5.2.1, PHP 7) / DateTime::format -- DateTimeImmutable::format -- DateTimeInterface::format -- date_format — Returns date formatted according to given format

The PHP documentation is copyrighted as such:

Copyright © 1997 - 2020 by the PHP Documentation Group. This material may be distributed only subject to the terms and conditions set forth in the Creative Commons Attribution 3.0 License or later. A copy of the Creative Commons Attribution 3.0 license is distributed with this manual. The latest version is presently available at » http://creativecommons.org/licenses/by/3.0/.