Get image alt attribute just by image URL

Question

Is there a possible way to get the alt text from the media file by just knowing the URL? For example, if the path to my image is /bc/wp-content/uploads/placeholder-image.png is there a way (just by using that URL) to get the alt text of that image. I know you usually need the ID and such, trying to find a way around this.

PHP

<img src="/bc/wp-content/uploads/placeholder-image.png" alt="<?php /* do something */ ?>" />

Answer 1

Using the function found here, you could add this to your functions.php

// retrieves the attachment ID from the file URL
function pippin_get_image_id($image_url) {
    global $wpdb;
    $attachment = $wpdb->get_col($wpdb->prepare("SELECT ID FROM $wpdb->posts WHERE guid='%s';", $image_url )); 
    return $attachment[0]; 
}

But to use it, you need the full image URL.

$img_url = get_bloginfo('url') . '/bc/wp-content/uploads/placeholder-image.png';

if ( $img_id = pippin_get_image_id($img_url) ) {
    // Using wp_get_attachment_image should return your alt text added in the WordPress admin.
    echo wp_get_attachment_image( $img_id, 'full' );
} else {
    // Fallback in case it's not found.
    echo '<img src="' . $img_url . '" alt="" />';
}

Answer 2

This is a slightly modified version of the Pippin Function. Since it returns a post ID we can use it to get the alternative text which is saved as Post Meta:

/**
 * Get image alt text by image URL
 *
 * @param String $image_url
 *
 * @return Bool | String
 */
function image_alt_by_url( $image_url ) {
    global $wpdb;

    if( empty( $image_url ) ) {
        return false;
    }

    $query_arr  = $wpdb->get_col( $wpdb->prepare( "SELECT ID FROM {$wpdb->posts} WHERE guid='%s';", strtolower( $image_url ) ) );
    $image_id   = ( ! empty( $query_arr ) ) ? $query_arr[0] : 0;

    return get_post_meta( $image_id, '_wp_attachment_image_alt', true );
}