Swap a block cover image conditionally

Replace a WordPress Cover block image conditionally, using an ACF field in this example.

add_filter( 'render_block', function( $block_content, $block ) {

	// Cover Only
	if( $block['blockName'] != 'core/cover' ) {
		return $block_content;
	}

	// Require ACF
	if( ! function_exists( 'get_field' ) ) {
		return $block_content;
	}

	// Current Object
	$term = get_queried_object();

	// Try Single Post Banner
	$banner_image = get_field( 'banner_image', $term );

	// Special Field Types
	if( ! empty( $banner_image['url'] ) ) {
		$banner_image = $banner_image['url'];
	}

	// Replace Cover Image
	if( $banner_image ) {
		$block_content = str_replace(
			'https://mysite.com/wp-content/uploads/2021/12/Placeholder.jpg',
			$banner_image, $block_content
		);
	}

	// Return
	return $block_content;

}, 10, 2 );