function isJson($string) {
   json_decode($string);
   return json_last_error() === JSON_ERROR_NONE;
}

function isJson($string) {
    $obj = json_decode($string);
    return json_last_error() === JSON_ERROR_NONE && gettype($obj ) == "object";
}

这是有效的，对于数字不返回true

新的更新

如果JSON很长并且你不需要使用$obj，上面的解决方案就没有很好的性能

如果你只是想检查一下，最好使用下面的函数

function isJson($string) {
    if(is_numeric($string)) return false;
    json_decode($string);
    return json_last_error() === JSON_ERROR_NONE;
}

对亨里克的回答做一个简单的修改，以触及最需要的可能性。

(包括“{}和[]”)

function isValidJson($string) {
    json_decode($string);
    if(json_last_error() == JSON_ERROR_NONE) {

        if( $string[0] == "{" || $string[0] == "[" ) { 
            $first = $string [0];

            if( substr($string, -1) == "}" || substr($string, -1) == "]" ) {
                $last = substr($string, -1);

                if($first == "{" && $last == "}"){
                    return true;
                }

                if($first == "[" && $last == "]"){
                    return true;
                }

                return false;

            }
            return false;
        }

        return false;
    }

    return false;

}

我知道这个问题已经过时了，但是我想通知大家，json_validate()函数的RFC已经实现，并且将成为php 8.3的一部分

这种方法将是实现问题要求的最快和更有效的方法。

Github -实现代码

RFC - json_validate()

再扩展一下这个答案，下面的问题如何:

<?php

    $json = '[{"user_id":13,"username":"stack"},{"user_id":14,"username":"over"}]';
    //$json = '12';

    function isJson($string) {
        json_decode($string);
        if(json_last_error() == JSON_ERROR_NONE) {
            if(substr($string,0,1) == '[' && substr($string,-1) == ']') { return TRUE; }
            else if(substr($string,0,1) == '{' && substr($string,-1) == '}') { return TRUE; }
            else { return FALSE; }
        }
    }

    echo isJson($json);
?>

使用PHPBench和下面的类，得到了以下结果:

<?php

declare(strict_types=1);

/**
 * @Revs(1000)
 * @Iterations(100)
 */
class BenchmarkJson
{
    public function benchCatchValid(): bool
    {
        $validJson = '{"validJson":true}';
        try {
            json_decode($validJson, true, 512, JSON_THROW_ON_ERROR);
            return true;
        } catch(\JsonException $exception) {}
        return false;
    }

    public function benchCatchInvalid(): bool
    {
        $invalidJson = '{"invalidJson"';
        try {
            json_decode($invalidJson, true, 512, JSON_THROW_ON_ERROR);
            return true;
        } catch(\JsonException $exception) {}
        return false;
    }

    public function benchLastErrorValid(): bool
    {
        $validJson = '{"validJson":true}';
        json_decode($validJson, true);
        return (json_last_error() === JSON_ERROR_NONE);
    }

    public function benchLastErrorInvalid(): bool
    {
        $invalidJson = '{"invalidJson"';
        json_decode($invalidJson, true);
        return (json_last_error() === JSON_ERROR_NONE);
    }

    public function benchNullValid(): bool
    {
        $validJson = '{"validJson":true}';
        return (json_decode($validJson, true) !== null);
    }

    public function benchNullInvalid(): bool
    {
        $invalidJson = '{"invalidJson"';
        return (json_decode($invalidJson, true) !== null);
    }
}

6 subjects, 600 iterations, 6,000 revs, 0 rejects, 0 failures, 0 warnings
(best [mean mode] worst) = 0.714 [1.203 1.175] 1.073 (μs)
⅀T: 721.504μs μSD/r 0.089μs μRSD/r: 7.270%
suite: 1343ab9a3590de6065bc0bc6eeb344c9f6eba642, date: 2020-01-21, stime: 12:50:14
+---------------+-----------------------+-----+------+-----+------------+---------+---------+---------+---------+---------+--------+-------+
| benchmark     | subject               | set | revs | its | mem_peak   | best    | mean    | mode    | worst   | stdev   | rstdev | diff  |
+---------------+-----------------------+-----+------+-----+------------+---------+---------+---------+---------+---------+--------+-------+
| BenchmarkJson | benchCatchValid       | 0   | 1000 | 100 | 2,980,168b | 0.954μs | 1.032μs | 1.016μs | 1.428μs | 0.062μs | 6.04%  | 1.33x |
| BenchmarkJson | benchCatchInvalid     | 0   | 1000 | 100 | 2,980,184b | 2.033μs | 2.228μs | 2.166μs | 3.001μs | 0.168μs | 7.55%  | 2.88x |
| BenchmarkJson | benchLastErrorValid   | 0   | 1000 | 100 | 2,980,184b | 1.076μs | 1.195μs | 1.169μs | 1.616μs | 0.083μs | 6.97%  | 1.54x |
| BenchmarkJson | benchLastErrorInvalid | 0   | 1000 | 100 | 2,980,184b | 0.785μs | 0.861μs | 0.863μs | 1.132μs | 0.056μs | 6.54%  | 1.11x |
| BenchmarkJson | benchNullValid        | 0   | 1000 | 100 | 2,980,168b | 0.985μs | 1.124μs | 1.077μs | 1.731μs | 0.114μs | 10.15% | 1.45x |
| BenchmarkJson | benchNullInvalid      | 0   | 1000 | 100 | 2,980,184b | 0.714μs | 0.775μs | 0.759μs | 1.073μs | 0.049μs | 6.36%  | 1.00x |
+---------------+-----------------------+-----+------+-----+------------+---------+---------+---------+---------+---------+--------+-------+

结论:检查json是否有效的最快方法是返回json_decode($json, true) !== null)。