PHP checking error JSON

<?php
class JsonHandler {
 
    protected static $_messages = array(
        JSON_ERROR_NONE => 'No error has occurred',
        JSON_ERROR_DEPTH => 'The maximum stack depth has been exceeded',
        JSON_ERROR_STATE_MISMATCH => 'Invalid or malformed JSON',
        JSON_ERROR_CTRL_CHAR => 'Control character error, possibly incorrectly encoded',
        JSON_ERROR_SYNTAX => 'Syntax error',
        JSON_ERROR_UTF8 => 'Malformed UTF-8 characters, possibly incorrectly encoded'
    );
 
    public static function encode($value, $options = 0) {
        $result = json_encode($value, $options);
 
        if($result)  {
            return $result;
        }
 
        throw new RuntimeException(static::$_messages[json_last_error()]);
    }
 
    public static function decode($json, $assoc = false) {
        $result = json_decode($json, $assoc);
 
        if($result) {
            return $result;
        }
 
        throw new RuntimeException(static::$_messages[json_last_error()]);
    }
 
}
?>


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// Returns "Correctly thrown"
    assertException("Syntax error", function() {
    $string = '{"foo": {"bar": {"cool": NONUMBER}}}';
    $result = JsonHandler::decode($string);
});