No tuve suerte, se ve que algún disparate estoy haciendo.
 
Tengo 2 archivos: 
formulario.php 
<form method="POST" action="cpemail.php"> 
   <input type="text" name="variable_usuario" size="20"></p>
  <p><input type="text" name="variable_password" size="20"></p>
  <p><input type="submit" value="Enviar"><input type="reset" value="Restablecer"></p>
</form> 
Y cpemail.php 
<?php
$variable_usuario = $_POST['NEWUSER']; 
$variable_password=$_POST['PASSWORD'];    
##################################################  #############
# cPanel Email Account Creator 1.0
##################################################  #############
# Visit 
http://www.zubrag.com/scripts/ for updates
##################################################  ############# 
# You can pass following parameters in calling URL. They will
# override those specified below.
# user - new email user
# pass - password
# domain - email domain
# quota - email quota, Mb
# Example: cpemail.php?user=newuser&pass=password"a=50
##################################################  #############  
// cPanel info
$cpuser = '---'; // cPanel username
$cppass = '---'; // cPanel password
$cpdomain = '---'; // cPanel domain or IP
$cpskin = '---';  // cPanel skin. Mostly x or x2. 
// See following URL to know how to determine your cPanel skin
// 
http://www.zubrag.com/articles/deter...panel-skin.php 
// Default email info for new email accounts
// These will only be used if not passed via URL
$euser = 'newuser'; // email username (without @ and domain)
$epass = 'hispassword'; // email password
$edomain = 'mysite.com'; // email domain (usually same as cPanel domain above)
$equota = 20; // amount of space in megabytes 
##################################################  ############# 
# END OF SETTINGS
##################################################  #############  
function getVar($name, $def = '') {
  if (isset($_GET[$name]))
    return $_GET[$name];
  else 
    return $def;
} 
// check if overrides passed
$euser = getVar('user', $euser);
$epass = getVar('pass', $epass);
$edomain = getVar('domain', $edomain);
$equota = getVar('quota', $equota); 
// Create email account
$f = fopen ("http://$cpuser:$cppass@$cpdomain:2082/frontend/$cpskin/mail/doaddpop.html?email=$euser&domain=$edomain&passwor  d=$epass"a=$equota", "r");
if (!$f) {
  die('Cannot create email account. Possible reasons: "fopen" function allowed on your server, PHP is running in SAFE mode');
} 
// Check result
while (!feof ($f)) {
  $line = fgets ($f, 1024);
  if (ereg ("already exists!", $line, $out)) {
    die('Such email account already exists.');
  }
}
fclose($f); 
echo "Email account created."; 
?> 
No llegan los valores. El contenido del cpemail.php no lo hice yo, solo lo de arriba que me dijiste. 
Desde ya, muchas gracias.