How do I obtain url from File/Image/Video in php?

I have a custom type “document” with a F/I/V field “source”. These are pdf, and I need to create a shortcode to display the pdf with a viewer on user page.
I could create the shortcode which includes the pdf url manually for each new document, but that is cumbersome and not good if other admins have to do it.
So I created another field, “shortcode_document”, and a php snippet that should set that field’s value to the full shortcode, pulling the url from the “source” field when the pod is saved.
I used example code on pods site and wrote this:

add_action( 'pods_api_pre_save_pod_item_document', 'document_update_pdf_shortcode_on_save', 10, 3 );

function document_update_pdf_shortcode_on_save( $pieces, $is_new_item ) {

	// s'assurer que les champs sont actifs
	$fields = array( 'source', 'shortcode_document' );
	foreach( $fields as $field ) {
		if ( ! isset( $pieces[ 'fields_active' ][ $field ] ) ) {
			array_push ($pieces[ 'fields_active' ], $field );
		}
	}
	
	$source = '';
	// get value from source
	if ( isset( $pieces[ 'fields' ][ 'source' ] ) && isset( $pieces[ 'fields' ][ 'source' ][ 'value' ] ) ) {
		$source = $pieces[ 'fields' ][ 'source' ][ 'value' ];
	}
	// debug
	error_log(__FUNCTION__);
	$count = count($source);
	error_log($count);
	for ($i = 0; $i < $count; $i++) {
		error_log($i.': '.$source[$i].' | ');
	}
	// full value for shortcode_document:
	$shortcode_document_value = '[flipbook pdf=\''.$source[0].'\']';
	
	error_log( $shortcode_document_value );
	
	$pieces[ 'object_fields'][ 'shortcode_document' ][ 'value' ] = $shortcode_document_value;
	return $pieces;
}

If I try to access the value of “source” directly, log says “Array”. But the loop through the array displays only 1 element, and it is empty. I tried
source[‘ID’] as recommended here:
https://stackoverflow.com/questions/12727438/how-to-get-url-of-uploaded-file-in-pods-framework-2
but that only yields a warning: “Illegal string offset ‘ID’…”.
Suggestions?