PHP Fatal error: Call to undefined function json_decode()

This error usually occurs when you are trying to use the json_decode() function, which is used to decode a JSON string into a PHP variable, but the function is not available.

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There are a few possible reasons why the json_decode() function might not be defined:

1. The JSON extension is not enabled in your PHP configuration. To enable it, you will need to edit your php.ini file and uncomment the line that reads extension=json.

2. You are using an older version of PHP that does not include the json_decode() function. The function was added in PHP 5.2, so if you are using an older version, you will need to upgrade to a newer version of PHP.

3. You have a custom build of PHP that was compiled without the JSON extension. In this case, you will need to recompile PHP with the JSON extension enabled.

If none of these solutions work, it's possible that there is a problem with your PHP installation. You might want to try reinstalling PHP to see if that fixes the issue.