curl error message “Object of class stdClass could not be converted to string” when using google computeRoute

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Summary

This incident stemmed from a type‑mismatch bug in a PHP cURL integration with Google’s computeRoutes API. A developer attempted to pass a stdClass object directly into CURLOPT_POSTFIELDS, which only accepts strings, arrays, or URL‑encoded form data. The result was the runtime error: “Object of class stdClass could not be converted to string.”

Root Cause

The root cause was the use of:

  • json_decode(json_encode($array)) which produces a stdClass object
  • Passing that object directly into CURLOPT_POSTFIELDS
  • cURL attempting to convert the object to a string and failing

Key takeaway: CURLOPT_POSTFIELDS requires a JSON string, not a PHP object.

Why This Happens in Real Systems

This class of bug is extremely common because:

  • PHP’s json_decode() defaults to returning objects, not arrays
  • cURL silently accepts many types but not objects
  • Developers often assume cURL will auto‑serialize complex structures
  • Dynamic payload generation makes type drift easy to miss

Real-World Impact

These failures often cause:

  • Silent API request failures with no useful response body
  • Hard‑to‑debug production errors because the payload never reaches the API
  • Inconsistent behavior when switching between static and dynamic payloads
  • Misleading error messages that point to PHP internals instead of the API

Example or Code (if necessary and relevant)

Below is the correct way to send the dynamically generated payload.

$payload = json_encode($this->converToObjects());

$curl = curl_init();
curl_setopt_array($curl, [
    CURLOPT_URL => 'https://routes.googleapis.com/directions/v2:computeRoutes',
    CURLOPT_RETURNTRANSFER => true,
    CURLOPT_CUSTOMREQUEST => 'POST',
    CURLOPT_POSTFIELDS => $payload,
    CURLOPT_HTTPHEADER => [
        'Content-Type: application/json',
        'X-Goog-Api-Key: myPrivateKey',
        'X-Goog-FieldMask: *'
    ],
]);

$res = curl_exec($curl);
curl_close($curl);

How Senior Engineers Fix It

Experienced engineers apply a few consistent practices:

  • Always encode payloads explicitly using json_encode()
  • Never pass objects into cURL unless they are strings
  • Validate payloads by logging the JSON before sending
  • Use associative arrays (json_decode($json, true)) to avoid unwanted stdClass objects
  • Write small helpers to enforce consistent serialization

Why Juniors Miss It

This issue is easy for less experienced developers to overlook because:

  • PHP’s loose typing hides type errors until runtime
  • json_decode() returning objects by default is unintuitive
  • cURL’s error message does not clearly indicate the real problem
  • Static payloads work fine, so the dynamic version seems equivalent
  • They assume cURL will “figure it out” and serialize automatically

Bottom line: The fix is simple—always send a JSON string, never a PHP object.

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