media_upload_form_handler() WordPress Function

function media_upload_form_handler() {
	check_admin_referer( 'media-form' );

	$errors = null;

	if ( isset( $_POST['send'] ) ) {
		$keys    = array_keys( $_POST['send'] );
		$send_id = (int) reset( $keys );
	}

	if ( ! empty( $_POST['attachments'] ) ) {
		foreach ( $_POST['attachments'] as $attachment_id => $attachment ) {
			$post  = get_post( $attachment_id, ARRAY_A );
			$_post = $post;

			if ( ! current_user_can( 'edit_post', $attachment_id ) ) {
				continue;
			}

			if ( isset( $attachment['post_content'] ) ) {
				$post['post_content'] = $attachment['post_content'];
			}

			if ( isset( $attachment['post_title'] ) ) {
				$post['post_title'] = $attachment['post_title'];
			}

			if ( isset( $attachment['post_excerpt'] ) ) {
				$post['post_excerpt'] = $attachment['post_excerpt'];
			}

			if ( isset( $attachment['menu_order'] ) ) {
				$post['menu_order'] = $attachment['menu_order'];
			}

			if ( isset( $send_id ) && $attachment_id == $send_id ) {
				if ( isset( $attachment['post_parent'] ) ) {
					$post['post_parent'] = $attachment['post_parent'];
				}
			}

			/**
			 * Filters the attachment fields to be saved.
			 *
			 * @since 2.5.0
			 *
			 * @see wp_get_attachment_metadata()
			 *
			 * @param array $post       An array of post data.
			 * @param array $attachment An array of attachment metadata.
			 */
			$post = apply_filters( 'attachment_fields_to_save', $post, $attachment );

			if ( isset( $attachment['image_alt'] ) ) {
				$image_alt = wp_unslash( $attachment['image_alt'] );

				if ( get_post_meta( $attachment_id, '_wp_attachment_image_alt', true ) !== $image_alt ) {
					$image_alt = wp_strip_all_tags( $image_alt, true );

					// update_post_meta() expects slashed.
					update_post_meta( $attachment_id, '_wp_attachment_image_alt', wp_slash( $image_alt ) );
				}
			}

			if ( isset( $post['errors'] ) ) {
				$errors[ $attachment_id ] = $post['errors'];
				unset( $post['errors'] );
			}

			if ( $post != $_post ) {
				wp_update_post( $post );
			}

			foreach ( get_attachment_taxonomies( $post ) as $t ) {
				if ( isset( $attachment[ $t ] ) ) {
					wp_set_object_terms( $attachment_id, array_map( 'trim', preg_split( '/,+/', $attachment[ $t ] ) ), $t, false );
				}
			}
		}
	}

	if ( isset( $_POST['insert-gallery'] ) || isset( $_POST['update-gallery'] ) ) {
		?>
		<script type="text/javascript">
		var win = window.dialogArguments || opener || parent || top;
		win.tb_remove();
		</script>
		<?php

		exit;
	}

	if ( isset( $send_id ) ) {
		$attachment = wp_unslash( $_POST['attachments'][ $send_id ] );
		$html       = isset( $attachment['post_title'] ) ? $attachment['post_title'] : '';

		if ( ! empty( $attachment['url'] ) ) {
			$rel = '';

			if ( strpos( $attachment['url'], 'attachment_id' ) || get_attachment_link( $send_id ) == $attachment['url'] ) {
				$rel = " rel='attachment wp-att-" . esc_attr( $send_id ) . "'";
			}

			$html = "<a href='{$attachment['url']}'$rel>$html</a>";
		}

		/**
		 * Filters the HTML markup for a media item sent to the editor.
		 *
		 * @since 2.5.0
		 *
		 * @see wp_get_attachment_metadata()
		 *
		 * @param string $html       HTML markup for a media item sent to the editor.
		 * @param int    $send_id    The first key from the $_POST['send'] data.
		 * @param array  $attachment Array of attachment metadata.
		 */
		$html = apply_filters( 'media_send_to_editor', $html, $send_id, $attachment );

		return media_send_to_editor( $html );
	}

	return $errors;
}