Checking for alt text on images added to Wordpress content - php

Question

I want to require that Wordpress editors must add alt text to images.
The site uses the classic editor and images are added into the content editor. I have chosen to do this in php by checking if images have alt text when saving the post. If all images do not have alt tags, then prevent the post from being published and show a message in the admin.
Here is the code I have. My problem is that the code to return the alt text always returns an empty string. Can someone help me pin down what I'm doing wrong? Also, not averse to a different approach altogether...
add_action('save_post', 'check_content_for_images');
function check_content_for_images($post_id) {
$attachments = get_children( array(
'post_parent' => $post_id,
'post_type' => 'attachment',
'numberposts' => -1, // show all -1
'post_status' => 'inherit',
'post_mime_type' => 'image',
) );
// loop through images and check that it has an alt attribute
$counter = 0;
foreach ($attachments as $attachment_id => $attachment) {
// $img = wp_get_attachment_image($attachment_id, 'full');
$alt_text = get_post_meta($attachment_id, '_wp_attachment_image_alt', true);
if(empty($alt_text)) {
$counter++;
}
}
if ( count($attachments) > $counter) {
// set a transient to show the users an admin message
set_transient( "post_not_ok", "not_ok" );
// // unhook this function so it doesn't loop infinitely
remove_action('save_post', 'check_content_for_images');
// // update the post set it to draft
wp_update_post(array('ID' => $post_id, 'post_status' => 'draft'));
// // re-hook this function
add_action('save_post', 'check_content_for_images');
} else {
delete_transient( "post_not_ok" );
}
}
/**
* Do not show post published message
*/
add_filter( 'post_updated_messages', 'abc_remove_all_messages' );
function abc_remove_all_messages( $messages ) {
if ( get_option( 'abc_post_error' ) ) {
return array();
} else {
return $messages;
}
}
// add a admin message to inform the user the post has been saved as draft.
function showAdminMessages()
{
// check if the transient is set, and display the error message
if ( get_transient( "post_not_ok" ) == "not_ok" ) {
// Shows as an error message. You could add a link to the right page if you wanted.
showMessage("One or more of the images included in this post is missing ALT text. Please ensure all images have ALT text", true);
// delete the transient to make sure it gets shown only once.
delete_transient( "post_not_ok" );
}
}
add_action('admin_notices', 'showAdminMessages');
// function to display the errormessage
function showMessage($message, $errormsg = false)
{
if ($errormsg) {
echo '<div id="message" class="error">';
}
else {
echo '<div id="message" class="updated fade">';
}
echo "<p><strong>$message</strong></p></div>";
}