Call to undefined function Zend_Json_Expr()?

I need to JSONize some data without quotes around the values for Javascript purposes.

I am following examples like

$data = array(
    'onClick' => new Zend_Json_Expr('function() {'
              . 'alert("I am a valid javascript callback '
              . 'created by Zend_Json"); }'),
    'other' => 'no expression',
);
$jsonObjectWithExpression = Zend_Json::encode(
    $data,
    false,
    array('enableJsonExprFinder' => true)
);

and then returning the $jsonObjectWithExpression, but I get an exception

Fatal error: Call to undefined function Zend_Json_Expr() in ...

I'm pretty sure my version 1.10.8 supports this feature, since I can see the class in json/Expr.php

Do I need to import/include this class somehow? I haven't had to import any othe开发者_JAVA百科r Zend classes..

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You may use Zend_Autoloader or Zend_Application or just require_once('/path/to/Zend/Json/Expr.php')

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Are you sure you're using the new keyword?

The error message seems to state you're attempting to call a function named Zend_Json_Expr