Coding Standards: Replace alias PHP functions with the canonical names.

Using the canonical function name for PHP functions is strongly recommended, as aliases may be deprecated or removed without (much) warning.

This replaces all uses of the following:
* `join()` with `implode()`
* `sizeof()` with `count()`
* `is_writeable()` with `is_writable()`
* `doubleval()` with a `(float)` cast

In part, this is a follow-up to #47746.

Props jrf.
See #50767.
Built from https://develop.svn.wordpress.org/trunk@49193


git-svn-id: http://core.svn.wordpress.org/trunk@48955 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/upload.php

@@ -162,7 +162,7 @@ if ( $doaction ) {
 			$location = add_query_arg(
 				array(
 					'trashed' => count( $post_ids ),
-					'ids'     => join( ',', $post_ids ),
+					'ids'     => implode( ',', $post_ids ),
 				),
 				$location
 			);