Wordpress：在上传时将特色图片alt标记更改为给定的alt标记

I am giving alt tags for an image while uploading, but it is not taking the given image alt tags , it shows blank

i tried to add this filter but still not working

/* Register callback function for post_thumbnail_html filter hook */
add_filter( 'post_thumbnail_html', 'meks_post_thumbnail_alt_change', 10, 5 );

/* Function which will replace alt atribute to post title */
function meks_post_thumbnail_alt_change( $html, $post_id, $post_thumbnail_id, $size, $attr ) {

    $post_title = get_the_title();
    $html = preg_replace( '/(alt=")(.*?)(")/i', '$1'.esc_attr( $post_title ).'$3', $html );

    return $html;

}

The post content is displayed from loop-single.php file

<div class="entry-content">
    <?php the_content(); ?>
    <?php wp_link_pages( array( 'before' => '<div class="page-link">' . __( 'Pages:', 'twentyten' ), 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->

How to use those alt tags for my post featured image

Well i found out the error , it seems previous developer added the image manually to the post and featured image was also set . so i got confused.

i simply put the alt text in edit mode of the post directly.

Try this it worked for me :)

function change_post_thumbnail_html($html, $post_id, $post_thumbnail_id, $size, $attr) {
    $id = get_post_thumbnail_id();
    $src = wp_get_attachment_image_src($id, $size); 
    $alt = get_the_title($id); // gets the post thumbnail title
    $class = $attr['class'];
    $html = '<img src="' . $src[0] . '" alt="' . $alt . '" class="' . $class . '" />';
    return $html;
}
add_filter('post_thumbnail_html', 'change_post_thumbnail_html', 99, 5);