php use a variable in filter_input()

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PHP

I am trying to use a variable to pickup posts with filter_input()
The code is as follows:

for ($i='1'; $i<='10'; $i++)
{
      $childname = "ChildsName".$i;
      print "childname = $childname<br />";
      $ChildsName = filter_input(INPUT_POST,'$childname');
      print "$i. $ChildsName<br />\n";

}
I am positive that the post ChildsName1 is coming through as Eliza but it does not show in the above print statements. It comes back null.
The first print shows $childname to be ChildsName1 as it should be.

I don't want to have to repeat the code 10 times....

Thanks, your help is appreciated.
Hal