File: wp-includes/media.php

function wp_get_attachment_image_src( $attachment_id, $size = 'thumbnail', $icon = false ) {
    // Get a thumbnail or intermediate image if there is one.
    $image = image_downsize( $attachment_id, $size );
    if ( ! $image ) {
        $src = false;
 
        if ( $icon ) {
            $src = wp_mime_type_icon( $attachment_id );
 
            if ( $src ) {
                /** This filter is documented in wp-includes/post.php */
                $icon_dir = apply_filters( 'icon_dir', ABSPATH . WPINC . '/images/media' );
 
                $src_file               = $icon_dir . '/' . wp_basename( $src );
                list( $width, $height ) = wp_getimagesize( $src_file );
            }
        }
 
        if ( $src && $width && $height ) {
            $image = array( $src, $width, $height, false );
        }
    }
    /**
     * Filters the attachment image source result.
     *
     * @since 4.3.0
     *
     * @param array|false  $image         {
     *     Array of image data, or boolean false if no image is available.
     *
     *     @type string $0 Image source URL.
     *     @type int    $1 Image width in pixels.
     *     @type int    $2 Image height in pixels.
     *     @type bool   $3 Whether the image is a resized image.
     * }
     * @param int          $attachment_id Image attachment ID.
     * @param string|int[] $size          Requested image size. Can be any registered image size name, or
     *                                    an array of width and height values in pixels (in that order).
     * @param bool         $icon          Whether the image should be treated as an icon.
     */
    return apply_filters( 'wp_get_attachment_image_src', $image, $attachment_id, $size, $icon );
}

页面上调用
<?php 
	$url = wp_get_attachment_url( get_post_thumbnail_id(102) );
	echo $url
?>