Date/Time: Fix 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.                                                                                       
                  
  Corrects the substr() positions and mktime() argument order so that                                              
  variable names, comments, and logic are all consistent.

Author: saratheonline

src/wp-includes/functions.php

@@ -589,13 +589,13 @@ function get_weekstartend( $mysqlstring, $start_of_week = '' ) {
 	$my = substr( $mysqlstring, 0, 4 );
 
 	// MySQL string month.
-	$mm = substr( $mysqlstring, 8, 2 );
+	$mm = substr( $mysqlstring, 5, 2 );
 
 	// MySQL string day.
-	$md = substr( $mysqlstring, 5, 2 );
+	$md = substr( $mysqlstring, 8, 2 );
 
 	// The timestamp for MySQL string day.
-	$day = mktime( 0, 0, 0, $md, $mm, $my );
+	$day = mktime( 0, 0, 0, $mm, $md, $my );
 
 	// The day of the week from the timestamp.
 	$weekday = (int) gmdate( 'w', $day );