WordPress: (very specific use script) Matches number in attachment (PDF) name to a custom field value and attaches media to that post.

wp-attach-media-by-custom-field.php

<?php
/* Get unattached files in Media Library */
$args_search = array(
	'post_type'      => 'attachment',
	'post_status'    => 'inherit',
	'post_parent'    => 0,
	'post_mime_type' => 'application/pdf', // Only PDFs.
	'posts_per_page' => 40,
	'orderby'        => 'id',
	'order'          => 'ASC',
	's'              => 'trail_study_',
);
$query_search = new WP_Query( $args_search );


/* Make array unattached files' slugs (value) and IDs (keys) */
foreach ( $query_search->posts as $post ) {
	$attachments[$post->ID] = $post->post_name; // Make array.
	// echo "'$post->ID' => '$post->post_name',\n"; // Print array items.
}

// print_r( $attachments );

/* Find a post with attachment slug in content (e.g, in a link). */
foreach ( $attachments as $attach_id => $attach_name ) {

	// Get number from slug (e.g.: 'trail_study_5-nebraska-rural-trails').
	$slug_arr = preg_split( '/(\d+)\K/', $attach_name, -1, PREG_SPLIT_DELIM_CAPTURE );
						   
	$args = array(
		'post_type'             => 'trail',
		'meta_key'              => 'study_id',
		'meta_value'          => $slug_arr[1],
		'fields'                       => 'ids',
		'posts_per_page' => 1,
	);
	$query = new WP_Query( $args );

	$post_id = $query->posts[0];
	
  // For testing befoe=re running.
	echo "<pre>$attach_id\t$attach_name\t$slug_arr[1]\t$post_id</pre>";

	// Attach file (in Media Library) to post.
  // UNCOMMENT TO RUN
  // $wp_update_post = wp_update_post( array( 'ID' => $attach_id,  'post_parent' => $post_id ) );

  // UNCOMMENT TO PRINT RESULTS  
  // echo "$slug_arr[1]\t$attach_id\t$wp_update_post\rhttps://headwaterseconomics.org/trail/$post_slug/\r\r";
}

?>