src/Zend/Config/Yaml.php
protected static function _parseValue($value)
    {
        $value = trim($value);

        // remove quotes from string.
        if ('"' == $value['0']) {
            if ('"' == $value[count($value) -1]) {
                $value = substr($value, 1, -1);
            }
        } elseif ('\'' == $value['0'] && '\'' == $value[count($value) -1]) {
            $value = strtr($value, array("''" => "'", "'" => ''));
        }

        // Check for booleans and constants
        if (preg_match('/^(t(rue)?|on|y(es)?)$/i', $value)) {
            $value = true;
        } elseif (preg_match('/^(f(alse)?|off|n(o)?)$/i', $value)) {
            $value = false;
        } elseif (strcasecmp($value, 'null') === 0) {
            $value = null;
        } elseif (!self::$_ignoreConstants) {
            // test for constants
            $value = self::_replaceConstants($value);
        }

        return $value;
    }
protected static function _parseValue($value)
    {
        $value = trim($value);

        // remove quotes from string.
        if ('"' == $value['0']) {
            if ('"' == $value[count($value) -1]) {
                $value = substr($value, 1, -1);
            }
        } elseif ('\'' == $value['0'] && '\'' == $value[count($value) -1]) {
            $value = strtr($value, array("''" => "'", "'" => ''));
        }

        // Check for booleans and constants
        if (preg_match('/^(t(rue)?|on|y(es)?)$/i', $value)) {
            $value = true;
        } elseif (preg_match('/^(f(alse)?|off|n(o)?)$/i', $value)) {
            $value = false;
        } elseif (strcasecmp($value, 'null') === 0) {
            $value = null;
        } elseif (!self::$_ignoreConstants) {
            // test for constants
            $value = self::_replaceConstants($value);
        }

        return $value;
    }
Parse values

@param string $value
@return string