There has been a lot of Discussions about Enterprise PHP, specially after the release of PHP 5. PHP has a much better Object Model now, butt PHP is still NOT enterprise ready.
The first step toward enterprise is PHP's emancipation from Apache Web server. Since 4.1.0 Version, PHP supports sockets. Therefore PHP itself can serve HTTP Requests. Although PHP sockets are experimental, but it really rocks. I have written a simple IRC Sever in PHP using PHP sockets and it is up since about 70 days.
Read: PHP must free itself from Apache Server