Check STRING is JSON

function.php

<?php
/**
 * Check string is JSON
 * 
 * @param string $json JSON to check
 * 
 * @return boolean|array
 */
function is_json( $json = null, $_return = false ){
    if( ! is_string( $json ) ){
        return false;
    }

    $_json = @ json_decode( $json, false );
    $is_json = $_json != null;

    if( $_return !== true )
        return (bool) $is_json;

    return ( $is_json ) ? $_json : false;
}