I am about to buy an ecommerce application which is PHP powered for a
project at my corporation. I know that using PHP is unprofessional,
but I'm not sure exactly why. What makes ASP the choice of
corporations instead of PHP?

who says that PHP is unproffesional?

I write all my pages in ASP because I found it easier to learn, and
the MS environment is more familiar to me.

I've often heard it say that if you have never learnt to program
before then PHP is easier to learn.

They are both just two different ways of achieving the same thing.

PHP runs in a linux/unix environment while ASP runs in a Windows
environment (and yes before anyone jumps in, you can install software
to run either language in either environment, but the results are
never completely predictable)

MS hosting is generally more expensive than linus hosting too.

oh and let's not get into the .NET framework!!

PHP being unprofessional is more of a feeling that I have after
working at organizations ranging from dotcoms to Fortune 10
corporations. I've never seen a professional development group run
their applications in PHP.

But from all the feedback I'm getting, it sounds like the image that I
had isn't universal. It seems that most of those corps just used ASP
because it was easier to integrate with their other MS products.

Because it is not Microsoft.

severisth:
I'm just confirming the general consensus here with my own comments/experience:

ASP programmers are 2 or 3 times the cost of PHP programmers
PHP code is much easier for them to find; probably why it's cheaper
vBulletin is probably the most popular bulletin board software on the
web; lots of people probably learned coding by modifying it
Many PHP hosting gives bunches of free software

Interesting link on the subject:
http://www.tiobe.com/tpci.htm

ASP is the choice of corporations because they like Microsoft.

PHP is quite easy to learn, and is generally find in *nix
environments, but it is almost equally well supported on Windows. 
Linux servers tend to be cheaper, because there are no licensing
issues to run it, and the hardware demands for a Linux server are
less.  It could seem to you that it's "unprofessional" because it is
less expensive, but in no way is it less capable.

"What makes ASP the choice of corporations instead of PHP?"

It's not. As many coporations use Java or PHP as use ASP. 

"I know that using PHP is unprofessional"

It's not. But when the world's largest software company is spending
millions to impact your opinion it is not suprising that you make a
statement like that.

- PHP runs on both Unix and Windows servers. ASP does not. 

- PHP is available on the broadest variety of hosting plans (Unix and
Windows). ASP only on Windows.

- PHP has a large and active community of open source developers which
means there is a huge amount of code and applications available.

- As you found there are many powerful applications for e-commerce,
content management, etc. in PHP.

From the zend.com website:

"PHP is an open source Web development language. Along with the Apache
Web Server and Linux Operating System, PHP is among the most widely
deployed and used pieces of open source software. PHP, according to
NetCraft, has surpassed Microsoft ASP, making it the most popular Web
development language and is being utilized on over 15 million Websites
today.

PHP has proven itself across a range of large enterprise deployments
at companies worldwide including: Hewlett-Packard, Boeing, Lufthansa,
Dresdner Bank, Disney Online, Yahoo!, Lycos, Sprint, T-Mobile, Orange,
Nortel Networks, Lucent, WallStreetOnline and Siemens."