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function is not described

json_last_error_msg() WP 4.4.0

Retrieves the error string of the last json_encode() or json_decode() call.

No Hooks.

Return

true/false/String. Returns the error message on success, "No Error" if no error has occurred, or false on failure.

Usage

json_last_error_msg();

Code of json last error msg: wp-includes/compat.php VER 5.1.1

<?php
function json_last_error_msg() {
	// See https://core.trac.wordpress.org/ticket/27799.
	if ( ! function_exists( 'json_last_error' ) ) {
		return false;
	}

	$last_error_code = json_last_error();

	// Just in case JSON_ERROR_NONE is not defined.
	$error_code_none = defined( 'JSON_ERROR_NONE' ) ? JSON_ERROR_NONE : 0;

	switch ( true ) {
		case $last_error_code === $error_code_none:
			return 'No error';

		case defined( 'JSON_ERROR_DEPTH' ) && JSON_ERROR_DEPTH === $last_error_code:
			return 'Maximum stack depth exceeded';

		case defined( 'JSON_ERROR_STATE_MISMATCH' ) && JSON_ERROR_STATE_MISMATCH === $last_error_code:
			return 'State mismatch (invalid or malformed JSON)';

		case defined( 'JSON_ERROR_CTRL_CHAR' ) && JSON_ERROR_CTRL_CHAR === $last_error_code:
			return 'Control character error, possibly incorrectly encoded';

		case defined( 'JSON_ERROR_SYNTAX' ) && JSON_ERROR_SYNTAX === $last_error_code:
			return 'Syntax error';

		case defined( 'JSON_ERROR_UTF8' ) && JSON_ERROR_UTF8 === $last_error_code:
			return 'Malformed UTF-8 characters, possibly incorrectly encoded';

		case defined( 'JSON_ERROR_RECURSION' ) && JSON_ERROR_RECURSION === $last_error_code:
			return 'Recursion detected';

		case defined( 'JSON_ERROR_INF_OR_NAN' ) && JSON_ERROR_INF_OR_NAN === $last_error_code:
			return 'Inf and NaN cannot be JSON encoded';

		case defined( 'JSON_ERROR_UNSUPPORTED_TYPE' ) && JSON_ERROR_UNSUPPORTED_TYPE === $last_error_code:
			return 'Type is not supported';

		default:
			return 'An unknown error occurred';
	}
}