WordPress, MathJax使用のチェックボックス追加。 JavaScriptの挿入。

<?php
add_action('save_post', function ( $post_id ) {
	if( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ){
		return;
	}

	$meta_key = '_post_mathjax';
	$request_post = json_encode( $_POST[ $meta_key ] );
	if ( isset( $request_post ) ) update_post_meta( $post_id, $meta_key, $request_post );
});
add_action('add_meta_boxes', function() {
	add_meta_box('mathjax', __( 'MathJax設定', 'theme_name' ), 'post_mathjax', ['post', 'page'], 'side', 'high');
});

function post_mathjax() {
	global $post;
	$post_id = $post->ID;
	$meta_key = '_post_mathjax';
	$meta_value = get_post_meta( $post_id, $meta_key, true );
	$meta_array = [];
	if ( $meta_value ) $meta_array = json_decode( $meta_value, true );

	$name = $meta_key . '[enabled]';
	$checked = ( !empty( $meta_array ) && $meta_array['enabled'] === '1' ) ? 'checked' : '';
	?>
	<div>
	<input type="hidden" name="<?php echo $name; ?>" value="">
	<label for="meta_mathjax_enabled"><input type="checkbox" name="<?php echo $name; ?>" id="meta_mathjax_enabled" value="1"  <?php echo $checked; ?> /> <?php _e('MathJaxを使う', 'theme_name'); ?></label>
	</div>
	<?php
}

add_action( 'wp_enqueue_scripts', function(){
	global $post;
	$post_id = $post->ID;
	$meta_key = '_post_mathjax';
	$meta_value = get_post_meta( $post_id, $meta_key, true );
	$meta_array = [];
	if ( $meta_value ) $meta_array = json_decode( $meta_value, true );

	if ( empty( $meta_array ) ) return;
	if ( empty( $meta_array['enabled'] ) ) return;

	//$src= 'https://polyfill.io/v3/polyfill.min.js?features=es6';
	$src= 'https://polyfill.io/v3/polyfill.js?features=es6';
	wp_enqueue_script( 'MathJax-script-polyfill', $src, [], false, true );

	$src = 'https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-mml-chtml.js';
	wp_enqueue_script( 'MathJax-script', $src, ['MathJax-script-polyfill'], false, true );
} );

add_action( 'script_loader_tag', function( $tag, $handle, $src ) {
	$targets = [
		'MathJax-script',
	];
	if ( in_array( $handle, $targets, true ) ) {
		$tag = str_replace( '<script src=', '<script async src=', $tag );
	}

	return $tag;
}, 10, 3 );