Validating JSON API Input shows how to apply a focused PHP technique using understandable, runnable examples and clearly defined constraints.
What Problem Does This Solve?
Decode JSON with JSON_THROW_ON_ERROR, then validate the expected object shape, field types and size limits. Encode response values with JSON_THROW_ON_ERROR.
Core Concepts
- Identify the data entering the operation and the expected output.
- Decode JSON with JSON_THROW_ON_ERROR, then validate the expected object shape, field types and size limits. Encode response values with JSON_THROW_ON_ERROR.
- Handle expected failures and test boundary conditions rather than assuming success.
Example 1: A Small Working Example
Start with a small input so the behavior is easy to follow.
<?php
$payload = json_decode("{\"quantity\":3}", true, 512, JSON_THROW_ON_ERROR);
if (!is_int($payload["quantity"] ?? null) || $payload["quantity"] < 1) { throw new InvalidArgumentException("Invalid quantity"); }
echo $payload["quantity"];
JSON_THROW_ON_ERROR reports malformed JSON through an exception. Validate the resulting PHP values and required fields after decoding.
Example 2: A Different Case
The second example changes the task or data while using the same general technique.
<?php
$payload = json_decode("{\"email\":\"[email protected]\"}", true, 512, JSON_THROW_ON_ERROR);
if (!filter_var($payload["email"] ?? null, FILTER_VALIDATE_EMAIL)) { throw new InvalidArgumentException("Invalid email"); }
echo "Valid email";
JSON_THROW_ON_ERROR reports malformed JSON through an exception. Validate the resulting PHP values and required fields after decoding.
Practical Example
This practical example is a complete standalone PHP snippet to adapt to a real application.
<?php
$requestBody = "{\"items\":[{\"sku\":\"BOOK-1\",\"qty\":2}]}";
$data = json_decode($requestBody, true, 512, JSON_THROW_ON_ERROR);
if (!is_array($data["items"] ?? null)) { throw new InvalidArgumentException("Expected items array"); }
foreach ($data["items"] as $line) { if (!is_string($line["sku"] ?? null) || !is_int($line["qty"] ?? null)) { throw new InvalidArgumentException("Invalid item"); } }
echo "Validated";
JSON_THROW_ON_ERROR reports malformed JSON through an exception. Validate the resulting PHP values and required fields after decoding.
Important Behavior and Edge Cases
- JSON syntax validity does not prove the payload has the required fields or data types. Enforce body limits and validate the decoded structure.
- Decode JSON with JSON_THROW_ON_ERROR, then validate the expected object shape, field types and size limits. Encode response values with JSON_THROW_ON_ERROR.
- Test a malformed input, a missing dependency and a boundary value when those cases apply.
Comparison With Related Approaches
| Approach | Trade-off |
|---|---|
| Validating JSON API Input | Decode JSON with JSON_THROW_ON_ERROR, then validate the expected object shape, field types and size limits. Encode response values with JSON_THROW_ON_ERROR. |
| Related PHP approach | JSON is suitable for interoperable data; PHP serialize() is not appropriate for untrusted interchange. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Skipping a relevant boundary or security check | JSON syntax validity does not prove the payload has the required fields or data types. Enforce body limits and validate the decoded structure. |
| Choosing a technique without considering its trade-off | JSON is suitable for interoperable data; PHP serialize() is not appropriate for untrusted interchange. |
| Assuming sample data behaves like production input | Test missing fields, unexpected types, failures and the application’s actual trust boundaries. |
Key Takeaways
- Decode JSON with JSON_THROW_ON_ERROR, then validate the expected object shape, field types and size limits. Encode response values with JSON_THROW_ON_ERROR.
- JSON is suitable for interoperable data; PHP serialize() is not appropriate for untrusted interchange.
- Write tests around the inputs and failure cases most likely in your application.
Use these examples as a starting point and adapt the validations and boundaries to the specific application.