Docs: Correct swapped variable names and comments in get_weekstartend().

The variables `$mm` and `$md` had their `substr()` positions and inline comments swapped — `$mm` was extracting the day digits (position 8) while `$md` was extracting the month digits (position 5), contrary to what the comments indicated.

The output was accidentally correct because the two mistakes cancelled each other out in the `mktime()` call, but the misleading naming posed a future maintenance risk.

This commit corrects the `substr()` positions and `mktime()` argument order so that variable names, comments, and logic are all consistent.

Follow-up to [8598], [28918].

Props saratheonline, westonruter.
Fixes #65046.

git-svn-id: https://develop.svn.wordpress.org/trunk@62421 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/functions.php

@@ -586,16 +586,16 @@ function human_readable_duration( $duration = '' ) {
  */
 function get_weekstartend( $mysqlstring, $start_of_week = '' ) {
 	// MySQL string year.
-	$my = substr( $mysqlstring, 0, 4 );
+	$mysql_year = substr( $mysqlstring, 0, 4 );
 
 	// MySQL string month.
-	$mm = substr( $mysqlstring, 8, 2 );
+	$mysql_month = substr( $mysqlstring, 5, 2 );
 
 	// MySQL string day.
-	$md = substr( $mysqlstring, 5, 2 );
+	$mysql_day = substr( $mysqlstring, 8, 2 );
 
 	// The timestamp for MySQL string day.
-	$day = mktime( 0, 0, 0, $md, $mm, $my );
+	$day = mktime( 0, 0, 0, $mysql_month, $mysql_day, $mysql_year );
 
 	// The day of the week from the timestamp.
 	$weekday = (int) gmdate( 'w', $day );