Well seeing as how the PHP-.NET bridge implementations I've seen thus far utilize certain pieces of COM to load the .NET runtime then, so long as mono can integrate with the Cross-Platform COM engine (FireFox uses it), I see no reason for this to be regarded as impractical.
I. Funzioni .NET
Questo modulo è SPERIMENTALE. Ovvero, il comportamento di queste funzioni, i nomi di queste funzioni, in definitiva tutto ciò che è documentato qui può cambiare nei futuri rilasci del PHP senza preavviso. Siete avvisati, l'uso di questo modulo è a vostro rischio.
- dotnet_load -- Carica un modulo DOTNET
A Phalanger Mono version is in progeress:
Do note though that Phalanger doesn't run on Mono yet, and isn't likely to run fully under Mono in foreseable future (see: http://www.php-compiler.net/FAQ.htm) which probably defies the purpose of it all for non-Windows users. Seems to me that, due to architectural reasons and Google endorsment, php4mono is probably closer to being the way to go Php-Net-wyse.
See also Phalanger (http://www.php-compiler.net/), which seems to be similar to php4mono, except a lot more complete (Virtually everything in PHP is supported with very few exceptions, most scripts run unmodified, and all the PHP built in functions are implemented).
It lets you use any part of the .NET framework in your PHP app as if PHP were a managed language. And as an example, phpmyadmin runs without modifying the code at all (although of course it doesn't use any .NET stuff).
Jayesh Jain article can be found here (the link above didn't work for me):
Sterling has been working on the ability to use Mono with PHP which essentially allows .NET and more information can be seen here:
www.gotdotnet.com has lots of goodies
A nice initial reference site I ran across to learn about dotnet-related functionality is http://www.andymcm.com/dotnetfaq.htm