If you are viewing the result in a browser, you would need to view the page source code. It would show:
Browser hides everything between < and > considering it as a tag.
$op = $_REQUEST['op'] = 'crawlproc';
echo 'This tool can be executed in command line mode only';
$dV_0jUzLL8eOC2vrtAn = true;
$_REQUEST['bg'] = true;
$_REQUEST['resume'] = true;
Or if you really need to see the result in the browser, you could encode HTML special characters and echo the string:
Still could you tell me please: what generally you are trying to do ? Why are you storing the PHP code as a base64-encoded string ? If you are executing it later by eval(), please be very careful. eval() is a very dangerous function. It is necessary to be absolutely sure there is no way someone could inject his own code there.
$str = (base64_decode('Pz48P3BocAokb3AgPSAkX1JFUVVFU1RbJ29wJ10gPSAnY3Jhd2xwcm9jJzsKaWYoaXNzZXQoJF9TRVJWRVJbJ1JFUVVFU1RfTUVUSE9EJ10pKQp7CmVjaG8gJ1RoaXMgdG9vbCBjYW4gYmUgZXhlY3V0ZWQgaW4gY29tbWFuZCBsaW5lIG1vZGUgb25seSc7CmV4aXQ7Cn0KJGRWXzBqVXpMTDhlT0MydnJ0QW4gPSB0cnVlOwpjaGRpcihkaXJuYW1lKF9fRklMRV9fKSk7CiRfUkVRVUVTVFsnYmcnXSA9IHRydWU7CiRfUkVRVUVTVFsncmVzdW1lJ10gPSB0cnVlOwppbmNsdWRlICcuL2luZGV4LnBocCc7Cj8+'));
echo '<pre>' . htmlspecialchars($str) . '</pre>';