PHP AJAX请求报错SyntaxError: Unexpected token < in JSON at position 0求助

Let's break down exactly why you're hitting this error and walk through how to fix it.

The Root Cause #

Your error stems from one critical issue: the response coming from your PHP script isn't pure valid JSON. As you noted, the returned content starts with stray <? ?> characters before the JSON:

<? ?>{"Status":"Login Success"}

When jQuery tries to parse this response as JSON, the < at position 0 is invalid (JSON must start with { or [), so it throws the syntax error you're seeing.

Why This Happens #

That stray <? ?> is being sent as raw text to the client, which means one of two scenarios is occurring:

1. Short open tags are disabled on your server: PHP isn't parsing the <? ?> tag, so it's passed directly to the request instead of being processed.
2. An included file has extra output: One of the files you're requiring (Global.php, dbConnect.php, Studio7WebClass.php) likely contains an empty <? ?> tag, or whitespace/blank lines outside of PHP tags that get sent before your JSON content.

Step-by-Step Fixes #

1. Clean Up Your Included Files #

Go through each of the PHP files you're importing and make these changes:

• Remove any empty <? ?> tags entirely—they serve no functional purpose and are causing this output issue.
• Omit the closing ?> tag at the end of each file. PHP doesn't require this tag, and leaving it out prevents accidental whitespace or newlines after the tag from being sent as unwanted output.
• Ensure there's no whitespace, newlines, or HTML content before the opening <?php tag in any of these files.

2. Add a Safety Net for JSON Output #

Even after cleaning up your files, adding a buffer clear before echoing your JSON can catch any accidental leftover output. Insert ob_clean(); right before you send the final JSON:

// Clear any buffered output to ensure pure JSON
ob_clean();
echo $finalJSON;

3. Verify Your Response #

Test your PHP script directly (use tools like Postman, curl, or even just visit the script URL with a simulated POST request) to confirm the response is only valid JSON with no extra characters. It should look exactly like this:

{"Status":"Login Success"}

Bonus: Check Short Open Tag Settings (Optional) #

If you have a specific need for short tags, you can enable short_open_tag = On in your php.ini file. However, it's generally better to stick with full <?php tags to avoid cross-server compatibility issues.

内容的提问来源于stack exchange，提问作者cmdevelopmentuk