validate if a path is a subfolder of WP_CONTENT_DIR

functions.php

/**
 * Validate that a given path is a child of the WP_CONTENT_DIR
 * @param  string $path path to check
 * @return boolean	true/false
 */
function sav_path_is_valid( $path ) {
	if ( !defined( WP_CONTENT_DIR ) )
		return false;

	$start = 0;
	$end = strlen( WP_CONTENT_DIR );
	if ( substr( realpath( $path ), $start, $end ) != WP_CONTENT_DIR )
		return false;
	return true;
}