JSON error handling

Learn to catch JSON errors with json_last_error() and json_last_error_msg().

Catch errors with last_error

When the decode function receives an invalid string, it returns null and silently sets an error flag. You read this error as a code with last_error, and as a short message with last_error_msg.

If there is no error, last_error returns the ERROR_NONE constant. On a syntax error, ERROR_SYNTAX is returned and the message is Syntax error. So checking the state after decoding is a good habit.

Throw exceptions with THROW_ON_ERROR

In the traditional approach, you check the error with last_error. A modern alternative is the THROW_ON_ERROR flag. When passed to decode or encode, this flag makes the function throw an exception on error.

With this, you catch the error in one place using a try/catch block. You pick one approach: either check last_error after each call, or use THROW_ON_ERROR with try/catch.

Exercise Print the error message of an invalid string

The $text variable holds an invalid string. Decode it with the decode function, then print the error message with last_error_msg.

Show example solution

Decode $text, then call last_error_msg() with echo.
Example solution:
decode($text, true);
echo last_error_msg();

Test yourself

Match error functions and constants

json_last_error

returns the error code
json_last_error_msg

returns the error message
ERROR_NONE

no error state
ERROR_SYNTAX

syntax error
Which function returns the error message as text?
- the encode function
- the decode function
- the last_error function
- the last_error_msg function
Show answer

the last_error_msg function — Correct. The last_error_msg function returns the error message as a short text.

Check the error code

$text = '{invalid}';
decode($text, true);
if (____ !== ERROR_NONE) {
    echo last_error_msg();
}

Show answer

$text = '{invalid}';
decode($text, true);
if (last_error() !== ERROR_NONE) {
    echo last_error_msg();
}

Next lesson: Content-Type with header()

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