Logging In Application Via PHP - Cookie / POST Value?

I need to login into a certain website before I can attempt a data download. I am trying to do that via a PHP script. The login page is located at: [URL] Which has a form like this (extraneous elements removed):

<form name="Form1" method="post" action="login.aspx" id="Form1">
<input type="hidden" name="__EVENTTARGET" value="" />
<input type="hidden" name="__EVENTARGUMENT" value="" />
<input type="hidden" name="__VIEWSTATE" value="dDw1MTU3NTkxNTI7O2w8Y2hrYm94Oz4+08TlRVm+gb75yz3dIctChP3qf/E=" />
<script language="javascript" type="text/javascript">
<!--
function __POSTBack(eventTarget, eventArgument) {
var thisform;
if (window.navigator.appName.toLowerCase().indexOf("microsoft") > -1) {
thisform = document.Form1;
}
else {
thisform = document.forms["Form1"];
}
thisform.eventTarget.value = eventTarget.split("$").join(":");
thisform.eventArgument.value = eventArgument;
thisform.submit();
}
// -->
</script>
<input name="text_userid" id="text_userid" type="text" />
<input name="text_password" type="password" maxlength="30" id="text_password" />
<input name="chkbox" id="chkbox" type="checkbox" value="checkbox" />
<a id="submit" href="javascript:__POSTBack('submit','')"><img src="images/login.jpg"></a>
</form>

I'm trying to login into it via PHP, but I'm not sure what I'm doing wrong. Here's what I'm doing:............