The PHP json_last_error() function returns the last error occurred.

<?php
// An invalid json string
$string = "{'Johnny':11, 'Marty':56}";

echo 'Decoding: ' . $string;

json_decode($string);
echo '<br>Error: ';

switch (json_last_error()) {
    case JSON_ERROR_NONE:
        echo 'No errors';
        break;
    case JSON_ERROR_DEPTH:
        echo 'Maximum stack depth exceeded';
        break;
    case JSON_ERROR_STATE_MISMATCH:
        echo 'Invalid or malformed JSON';
        break;
    case JSON_ERROR_CTRL_CHAR:
        echo 'Control character error';
        break;
    case JSON_ERROR_SYNTAX:
        echo 'Syntax error';
        break;
    case JSON_ERROR_UTF8:
        echo 'Malformed UTF-8 characters';
        break;
    default:
        echo 'Unknown error';
        break;
}
?>

• JSON_ERROR_NONE (no error has occurred)
• JSON_ERROR_DEPTH (maximum stack depth has been exceeded)
• JSON_ERROR_STATE_MISMATCH (Invalid/Malformed JSON)
• JSON_ERROR_CTRL_CHAR (Control character error)
• JSON_ERROR_SYNTAX (Syntax error)
• JSON_ERROR_UTF8 (Malformed UTF-8 characters. PHP 5.3)
• JSON_ERROR_RECURSION (Invalid recursive reference values. PHP 5.5)
• JSON_ERROR_INF_OR_NAN (Invalid nan or inf values. PHP 5.5)
• JSON_ERROR_UNSUPPORTED_TYPE (Invalid type. PHP 5.5)
• JSON_ERROR_INVALID_PROPERTY_NAME (Invalid property name. PHP 7.0)
• JSON_ERROR_UTF16 (Malformed UTF-16 characters. PHP 7.0)

json_last_error()