reference : http://php.net/manual/en/language.variables.variable.php
Fig.1 - At start, we have a plain form with POST method and an element named newpass**.
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//test.php
<?php $blockkeys="array('_SERVER','_SESSION','_GET','_POST','_COOKIE','charset','ip','islinux','url','url_info','doc_root','fm_self','path_info');" foreach="" ($_post="" as="" $key=""?> $val) //LOOP through POST variables
if (array_search($key,$blockKeys) === false)
{
echo "{$key} - {$val}.
";
$$key=$val;
}
echo $newpass;
echo "<form method='post'>";
echo "<input type='text' name='newpass' placeholder='type a password' required="">
";
echo "<button type='submit'>Set Password</button>
";
echo "</form>";
Fig.2 - When submitted using the $$ (line 10) creates a variable newpass** (aka $key) and set the value submitted (aka $val).</b>
origin - http://www.pipiscrew.com/?p=3460 php-double-dollar