Live/production

<?php
list($first,$host) = explode(".",$_SERVER["SERVER_NAME"]);

$var ="<iframe width='100%' height='100%' style='height: 100vh; border: none;' src='https://appName.bubbleapps.io/$first'></iframe>";
$var .= "<style>";
$var .= "body {margin: 0;}";
$var .= "</style>";
echo $var;

?>
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Testing:
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<?php
list($first,$host) = explode(".",$_SERVER["SERVER_NAME"]);

$var ="<iframe width='100%' height='100%' style='height: 100vh; border: none;' src='https://appName.bubbleapps.io/version-test/$first'></iframe>";
$var .= "<style>";
$var .= "body {margin: 0;}";
$var .= "</style>";
echo $var;

?>