The tor process exposes a SOCKS proxy. SOCKS is a pretty simple protocol, so it's pretty easy to create a TCP connection (HTTP or otherwise) through tor from php. This code isn't a generic SOCKS client -- it relies heavily on the way that tor happens to behave.

$domain = 'en.wikipedia.org';
$port = 80;
$path = '/';
$socket = fsockopen('localhost', 9050);

if ($socket)
{
   fwrite($socket, "\x05\x01\x00");
   fread($socket, 2);
   fwrite($socket, "\x05\x01\x00\x03".chr(strlen($domain)));
   fwrite($socket, $domain);
   fwrite($socket, pack('n', $port));
   fread($socket, 10);
   fwrite($socket, 'GET '.$path
         . " HTTP/1.0\r\n"
         . "Host: $domain\r\n"
         . "Connection: Close\r\n\r\n");
   $string = '';
   while ($socket && !feof($socket))
   {
      $string .= fread($socket, 4098);
   }
   fclose($socket);

   echo $string;

   echo "\n";
}