You could set the textbox name to an array:
<input type="text" name="textboxes[]" />
<input type="text" name="textboxes[]" />
<input type="text" name="textboxes[]" />
<input type="text" name="textboxes[]" />
and then in the code
if (is_array($_GET["textboxes"])){
foreach ($_GET["textboxes"] AS $value){
echo $value." entered in a textbox.<br />";
}
}
edit: If you can not change the name, you could iterate over ALL GEt-values:
foreach ($_GET AS $key=>$value){
if (strpos($key, "Textbox") === 0){
echo $value." has been entered in textbox ".$key."<br />";
}
}