<?php

$jsontest = '{"name": "yasamin", "age": 25, "city": "shiraz"}';

function isValidJson($str)

 {

    json_decode($str);

    return (json_last_error() == JSON_ERROR_NONE);

}

if (isValidJson($jsontest)) {

    echo "The JSON string is valid

";

} else {

    echo "The JSON string is invalid

";

}