You need to put the code between the

<<<EOF

EOF;

So you should be

Code
/* PHP CODE HERE */
$body = <<<EOF

$bannerAd[1] = 'http://www.germanshepherdhome.net/testadbanner1.jpg';
$bannerAd[2] = 'http://www.germanshepherdhome.net/testadbanner2.jpg';
$bannerAd[3] = 'http://www.germanshepherdhome.net/testadbanner3.jpg';

$adCount = count($bannerAd);
$randomAdNumber = mt_rand(1, $adCount);
echo $bannerAd[$randomAdNumber];
EOF;