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-admin/theme-editor.php

@@ -301,7 +301,7 @@ else :
 				<?php if ( is_child_theme() && $theme->get_stylesheet() === get_template() ) : ?>
 					<div class="notice notice-warning inline">
 						<p>
-							<?php if ( is_writeable( $file ) ) : ?>
+							<?php if ( is_writable( $file ) ) : ?>
 								<strong><?php _e( 'Caution:' ); ?></strong>
 							<?php endif; ?>
 							<?php _e( 'This is a file in your current parent theme.' ); ?>
@@ -309,7 +309,7 @@ else :
 					</div>
 				<?php endif; ?>
 			</div>
-			<?php if ( is_writeable( $file ) ) : ?>
+			<?php if ( is_writable( $file ) ) : ?>
 				<p class="submit">
 					<?php submit_button( __( 'Update File' ), 'primary', 'submit', false ); ?>
 					<span class="spinner"></span>