Update php.html.markdown

Moved to group with associative arrays.

php.html.markdown

@@ -142,13 +142,13 @@ $associative = ['One' => 1, 'Two' => 2, 'Three' => 3];
 
 echo $associative['One']; // prints 1
 
+// Add an element to an associative array
+$array['Four'] = 4;
+
 // List literals implicitly assign integer keys
 $array = ['One', 'Two', 'Three'];
 echo $array[0]; // => "One"
 
-// Add an element to an associative array
-$array['Four'] = 4;
-
 // Add an element to the end of an array
 $array[] = 'Four';
 // or