Replace broken images in post content with a default image - Useful for dev environments

gistfile1.php

add_filter( 'the_content', function( $content ) {

    $content_path = ( defined( 'IMAGE_REPLACER_WP_CONTENT_PATH' ) ) ? IMAGE_REPLACER_WP_CONTENT_PATH : WP_CONTENT_DIR;

	$content_url = ( defined( 'IMAGE_REPLACER_WP_CONTENT_URL' ) ) ? IMAGE_REPLACER_WP_CONTENT_URL : WP_CONTENT_URL;

	preg_match_all( '/<img(?:\s|.)+?src="((?:\s|.)+?)"(?:\s|.)+?\/?>/', $content, $matches );

	if ( empty( $matches[1] ) )
		return $content;

	foreach ( $matches[1] as $match ) {

		$file_path = str_replace( $content_url, $content_path, $match );

		if ( file_exists( $file_path ) )
			continue;

		$content = str_replace( $match, ( defined ( 'IMAGE_REPLACER_DEFAULT_IMAGE_URL' ) ) ? IMAGE_REPLACER_DEFAULT_IMAGE_URL : $content_url . '/default.jpg', $content );
	}

	return $content;

} );