set-image-caption-in-wordpress.php

<?php

// The IDs of the posts for which you want to update the featured image caption
$post_ids_to_update = array( 21, 57, 95 );

// Loop through each of the post IDs
foreach ( $post_ids_to_update as $post_id ) {

	// Get the ID of this post's featured image
	$featured_image_id = get_post_thumbnail_id( $post_id );

	// Get the title of this post's featured image
	$featured_image_title = get_the_title( $featured_image_id );

	// Sanitize the text so we're ready to insert it into the database
	$new_caption_text = sanitize_text_field( $featured_image_title );

	// Set the new caption (a.k.a. the post excerpt) of the featured image
	wp_update_post( 'post_excerpt', $featured_image_title )
}