Why Does `json_decode()` Return NULL for Valid JSON?

json_decode() Returns NULL for Valid JSON: Troubleshooting

When attempting to decode JSON data using json_decode(), developers may encounter instances where the function returns NULL despite the JSON being seemingly valid. This issue can be particularly puzzling when the JSON passes validation against online JSON validators. To understand why this occurs and resolve it effectively, let's delve into the problem and its solution.

Invalid Characters in the JSON String

One common cause of this issue is the presence of invalid characters in the JSON string. While most JSON validators ignore them, PHP's json_decode() function can terminate abruptly when encountering such characters. These invalid characters typically range from 0x00 to 0x1F and 0x80 to 0xFF.

Solution: Removing Invalid Characters

To address this issue, you can use the preg_replace() function to remove all invalid characters from the JSON string before attempting to decode it. The following code demonstrates how to do this:

$json_string = preg_replace('/[\x00-\x1F\x80-\xFF]/', '', $json_string);

This regular expression removes all characters between 0x00 and 0x1F, as well as characters between 0x80 and 0xFF, effectively filtering out the invalid characters.

True Associative Arrays

Another issue that can cause json_decode() to return NULL is when you expect the result to be a true associative array, but PHP interprets it as an object. This can happen when the JSON string contains property keys that are not enclosed in double quotes.

To ensure that the result is a true associative array, you can use the second parameter of json_decode() and set it to true. This will force the result to be an associative array rather than an object. Here's an example:

$json_data = json_decode($json_string, true);

By employing these solutions, you can effectively resolve the issue of json_decode() returning NULL for seemingly valid JSON data, allowing you to successfully parse and utilize the JSON data in your PHP applications.

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