src/Zend/Filter/StringToUpper.php
public function filter($value)
    {
        if ($this->_encoding) {
            return mb_strtoupper((string) $value, $this->_encoding);
        }

        return strtoupper((string) $value);
    }
public function filter($value)
    {
        if ($this->_encoding) {
            return mb_strtoupper((string) $value, $this->_encoding);
        }

        return strtoupper((string) $value);
    }
Defined by Zend_Filter_Interface

Returns the string $value, converting characters to uppercase as necessary

@param  string $value
@return string