您必须验证您的输入，以确保您传递的字符串不是空的，实际上是一个字符串。空字符串不是有效的JSON。

function is_json($string) {
  return !empty($string) && is_string($string) && is_array(json_decode($string, true)) && json_last_error() == 0;
}

我认为在PHP中更重要的是确定JSON对象是否有数据，因为要使用数据，您将需要调用json_encode()或json_decode()。我建议拒绝空JSON对象，这样就不必在空数据上运行编码和解码。

function has_json_data($string) {
  $array = json_decode($string, true);
  return !empty($string) && is_string($string) && is_array($array) && !empty($array) && json_last_error() == 0;
}

在GuzzleHttp:

/**
 * Wrapper for json_decode that throws when an error occurs.
 *
 * @param string $json    JSON data to parse
 * @param bool $assoc     When true, returned objects will be converted
 *                        into associative arrays.
 * @param int    $depth   User specified recursion depth.
 * @param int    $options Bitmask of JSON decode options.
 *
 * @return mixed
 * @throws \InvalidArgumentException if the JSON cannot be decoded.
 * @link http://www.php.net/manual/en/function.json-decode.php
 */
function json_decode($json, $assoc = false, $depth = 512, $options = 0)
{
    $data = \json_decode($json, $assoc, $depth, $options);
    if (JSON_ERROR_NONE !== json_last_error()) {
        throw new \InvalidArgumentException(
            'json_decode error: ' . json_last_error_msg());
    }

    return $data;
}

/**
 * Wrapper for JSON encoding that throws when an error occurs.
 *
 * @param mixed $value   The value being encoded
 * @param int    $options JSON encode option bitmask
 * @param int    $depth   Set the maximum depth. Must be greater than zero.
 *
 * @return string
 * @throws \InvalidArgumentException if the JSON cannot be encoded.
 * @link http://www.php.net/manual/en/function.json-encode.php
 */
function json_encode($value, $options = 0, $depth = 512)
{
    $json = \json_encode($value, $options, $depth);
    if (JSON_ERROR_NONE !== json_last_error()) {
        throw new \InvalidArgumentException(
            'json_encode error: ' . json_last_error_msg());
    }

    return $json;
}

如果有本地文件站。Json无效，缺失，或超过一个月，做一些事情。

if (!is_array(json_decode(@file_get_contents("stations.json"))) || time() > filemtime("stations.json") + (60*60*24*31)){
  // The json file is invalid, missing, or is more than 1 month old
  // Get a fresh version
} else {
  // Up to date
}

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()

function is_json($str){ 
    return json_decode($str) != null;
}

当检测到非法编码时，http://tr.php.net/manual/en/function.json-decode.php返回值为空。

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)。