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.

git-svn-id: https://develop.svn.wordpress.org/trunk@49193 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/widgets/class-wp-widget-custom-html.php

@@ -281,7 +281,7 @@ class WP_Widget_Custom_HTML extends WP_Widget {
 					<# if ( data.codeEditorDisabled ) { #>
 						<p>
 							<?php _e( 'Some HTML tags are not permitted, including:' ); ?>
-							<code><?php echo join( '</code>, <code>', $disallowed_html ); ?></code>
+							<code><?php echo implode( '</code>, <code>', $disallowed_html ); ?></code>
 						</p>
 					<# } #>
 				<?php endif; ?>