src/PEAR/ErrorStack.php
function setMessageCallback($msgCallback)
    {
        if (!$msgCallback) {
            $this->_msgCallback = array(&$this, 'getErrorMessage');
        } else {
            if (is_callable($msgCallback)) {
                $this->_msgCallback = $msgCallback;
            }
        }
    }
function setMessageCallback($msgCallback)
    {
        if (!$msgCallback) {
            $this->_msgCallback = array(&$this, 'getErrorMessage');
        } else {
            if (is_callable($msgCallback)) {
                $this->_msgCallback = $msgCallback;
            }
        }
    }
Set an error code => error message mapping callback

This method sets the callback that can be used to generate error
messages for any instance
@param array|string Callback function/method