Why am I getting a 400 Bad Request error when calling OpenAI's ChatGPT API in PHP?

Below is a list of steps you can take to troubleshoot a 400 Bad Request error when calling the Chat Completions endpoint:

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1. Check the JSON Payload

A 400 error generally means that the API did not like your request. In your case, verify that the JSON payload is correctly formatted. For example, if the product name is unexpectedly empty or contains unexpected characters it could cause the request to be rejected.

To debug, try outputting the JSON payload before sending it:

PHPCopied
$jsonData = json_encode($data);
echo "Payload: $jsonData\n";

This will let you inspect if the "messages" array and other parameters are set correctly.

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2. Examine API Error Details

The API response may include details about what is wrong with the request. Since you're using file_get_contents, you can check the HTTP response headers and any error message returned by OpenAI. For instance:

PHPCopied
$response = file_get_contents($url, false, $context);

if ($response === FALSE) {
    $error = error_get_last();
    echo "Error calling OpenAI API.\n";
    print_r($http_response_header); // Headers may include error info
    print_r($error);
    return null;
}

This extra logging can help pinpoint if there’s an unexpected field or missing data.

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3. Ensure Headers and Authorization Are Correct

Verify your headers are exactly as expected by the API. The following headers are required:

• Content-Type: application/json
• Authorization: Bearer YOUR_API_KEY

Make sure that the API key is correct and that it’s included exactly with no additional spaces or formatting issues.

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4. Consider Switching to cURL for Better Error Handling

Using cURL instead of file_get_contents often provides more detailed error reports. Here’s an example of how you might call the API with cURL:

PHPCopied
function getOpenAIDescription($productName, $apiKey) {
    $url = "https://api.openai.com/v1/chat/completions";

    $data = [
        "model" => "gpt-4",
        "messages" => [
            [
                "role" => "system",
                "content" => "You are a helpful assistant generating product descriptions for liquor products. You are most interested in back of bottle labels, but will also consider tasting and other descriptive information. Limit the description to 200 words."
            ],
            [
                "role" => "user",
                "content" => "Provide a product description for: $productName"
            ]
        ],
        "max_tokens" => 150
    ];

    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($data));
    curl_setopt($ch, CURLOPT_HTTPHEADER, [
        "Content-Type: application/json",
        "Authorization: Bearer $apiKey"
    ]);

    $response = curl_exec($ch);
    if (curl_errno($ch)) {
        echo "Curl error: " . curl_error($ch) . "\n";
        curl_close($ch);
        return null;
    }
    $httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE);
    if ($httpCode >= 400) {
        echo "HTTP error Code: $httpCode\nResponse: $response\n";
        curl_close($ch);
        return null;
    }
    curl_close($ch);

    $responseData = json_decode($response, true);
    if (isset($responseData['choices'][0]['message']['content'])) {
        return trim($responseData['choices'][0]['message']['content']);
    } else {
        echo "Failed to parse OpenAI API response.\n";
        return null;
    }
}

This approach will deliver more detailed feedback if something is wrong with the request.

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5. Double-Check Your Variable Usage

Ensure that all your variables are defined correctly. For instance, your code snippet shows this line:

PHPCopied
tableName = 'XXX'; // Database table

It should have the dollar sign (i.e. $tableName) if you want to use it later. If the SQL query is referencing an undefined variable, it might affect your data flow (although it likely won’t trigger a 400 from OpenAI).

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Final Thoughts

A 400 error indicates that the request structure isn’t what the API expects. Checking the payload, headers, and any extra error details is the best way to narrow down the issue. If nothing appears wrong on your end, use more detailed debugging (as with cURL) to inspect the returned HTTP headers and error messages.

By following these steps, you should be able to identify and correct the source of the bad request error.