I don't see the reason why ask a girl out because you will never find a hot blond that knows much about PHP...
but if you do, here are the collection of scripts...
Note: Please don't do too much PHP or you end up like this.

The straight forward noob version

echo 'want to go out tonight?"

Lame but useful, can be understand by almost...wait, any girl who is not mentally retarded. So this is actually doing nothing but:
-fake you are geek(to real geeky girls)
-you are a amateur geek(to normal girls)
-you are cute(Japanese girls...if you are white...)

Procedural + Conditional
Yes, now you are talking, at least put some procedural and conditional blood in your work.

if('will you answer 2 of my next questions with "yes" or "no"?'){
	$question='if my next question is
	"will you go out with me?"
	will your answer same as the one for this question?';
	if($_REQUEST['answer'] == 'no' || $_REQUEST['answer'] == 'yes'){
		echo 'I know you will fall for this...';
	}else{
		echo 'do I have to make a input filter?';
	}
}

a user input filter is recommended. This should be the most suitable way to ask both geek girls and normal girls that understand English.

Functional
..not y = 3x function.. a function like this... but don't use this to the OO PHP girls...

function wana_go_out($result){
	if($result==1){
		return 'we meet at....';
	}else{
		return 'just kidding, you look fat in your pants...';
	}
}

OOP PHP
If you are asking some OOP PHP programmer girl out for a date, with a pure procedural script, you can never make it. Unless she is a noob and does not think like the normal people, which is "OOP PHPer should be smug asses when compare to procedural PHPer".
this is pretty easy.. just make wana_go_out function part of the class...

class wana_go_out{
	var $return;
function wana_go_out($result){
	if($result==1){
		$this->return = 'we meet at....';
	}else{
		$this->return = 'why I'm I doing OO for this crap...';
	}
}
}

Don't send to procedural PHP girls... those ones will think you are trying to follow the "new way of programming" for the gay people...

some other technique you have to know:
do Obfuscation to the code:
if you want to date extreme geeks who does not think variable name have to be make sense.
Encrypt the code:
yes.. those public key things and such hardcore security things... if you want to date Asian convenient store owner.
Make it into a PHP virus:
polymorphic virus that infect all PHP files and echo out what you want to say, and possibly make you get into jail. Only think about that when the girl like wild men.
Use Google Map show the dating location
Yes, I know Google Maps [Could Have] Killed James Kim. But MSN and Yahoo! are not any better.
CAPTCHA check
you never know if she is a robot. She might work with you and do almost anything normal. but it will never pass some powerful animated CAPTCHAs.
XML-RPC
XML-RPC her server with messages, if she have one.
Last but not least:
write a 500 Page documentation for setting up a Unix Server that runs PHP.
another 5000 pages for how to understand your code.
Because:

"Programming today is a race between software engineers
striving to build bigger and better idiot-proof programs
and the Universe trying to produce bigger and better
idiots. So far, the Universe is winning."
-- Rich Cook

IMPORTANT:
For the PHP Bloggers, please don't tell any girl to click on your adsense!! Because Google(and god) owns the world and it will make you never Google any decent girl again.