您可以尝试读取/json目录中的所有文件，并在文件列表中搜索以检查名称是否已被使用。
在这里我提供了一个小例子，我认为可以帮助您报告的问题：

<?php

//get the id form the $_GET Array or any other input
$file_id = $_GET['id_file'];

/**
 * So i create a filepath supposing that the json Dir is on the same level of the
 * php file, for convenience, in a rela world application this could be a problem in terms of security
 * if the json directory contains sensitive data.
 */
$jsonDir = __DIR__ . DIRECTORY_SEPARATOR . "json";

//We can use the scandir function to get the list of all the files presents in the json directory

$files = scandir($jsonDir);

//Now we can create the name of th json file, in this case combining the id received by the form and the json extension
$fileName = "$file_id.json";

/**
 *
 * if there aren't file int the directory or there is non file with the computed name
 * the filename is available.
 *
 */

if (count($files) < 1 || !in_array($fileName, $files)){
   //The filename is Available
    echo "Available";
}
else{
    //The filename is not Available
    echo "Not Available";
}

exit;