Add a query string to the popup's URL giving values to the variables (note, omit $):
script.php3?a=1&b=test&c=foo
will pass $a, $b and $c to script.php3 when it is parsed.
You can't "open a popup in PHP"; what you do is emit the code to open one on the client browser. e.g.:
<?
$var = "abcd";
echo "<script> window.open('script.php3?var=",rawurlencode($var),"','title'); </script>";
?>
(This also passes the $var variable into script.php3, which is parsed to generate the popup's contents. rawurlencode() correctly escapes special characters - such as space, & itself, etc - which might be in $var.)
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