I am trying to write a script that fills out forms automatically and then automatically presses the submit button.
I have read that you can use curl to post HTTP requests, but what do you do when the form handles the post request with JavaScript, like the code below?
<form name="myform" action="javascript:void(0)" method="POST" onsubmit="return fancy_function(this)">
Based off your answer to the comments above, what you want to do is not "fill out the form" with curl, but rather post the completed form to the same page the form would normally send to.
This is also called cross-site posting, and many developers specifically test for and don't allow it to improve security. This will also be much much harder if the form has a captcha.
Assuming that it still makes sense to do (I've used this technique before for a newsletter signup form), here's what you want to do:
To get the form's action url, you'll need to look through the Javascript code for the site and find where funcy_function() is defined. Then in the body of the function, you'll likely see the destination url. You'll also need to note the specific names of the form variables in the html. You'll need to setup your variables with the exact same names. Then your curl setup will look like this:
$url = 'http://www.targeturl.com';
$myvars = 'myvar1=' . $myvar1 . '&myvar2=' . $myvar2;
$ch = curl_init( $url );
curl_setopt( $ch, CURLOPT_POST, 1);
curl_setopt( $ch, CURLOPT_POSTFIELDS, $myvars);
curl_setopt( $ch, CURLOPT_FOLLOWLOCATION, 1);
curl_setopt( $ch, CURLOPT_HEADER, 0);
curl_setopt( $ch, CURLOPT_RETURNTRANSFER, 1);
$response = curl_exec( $ch );
This will send the post variables to the specified url, imitating what would normally happen from the form. The html that the page returns will be in $response if you want to parse or use it.
If you want to use cURL:
You can capture every HTTP request (GET or POST forms) with Firefox Web-Tools:
Web-Tools -> Network -> send your form -> context menu of the form URL (right click on form script) -> Copy => copy as cURL address
More infos: https://everything.curl.dev/usingcurl/copyas
