How to make a PHP encryption code with the decryption code?

function decrypt($a){
$b=strtolower(strrev($a));
$c=hexdec(substr($b,0,4)) ^ hexdec('BFF') ^ hexdec('D77D');
$d=substr($b,4);
unset($p);
for($k=0;$k<strlen($d);$k+=6){
++$m;//!6
$e=($m*$m)^ hexdec('6E');//!7
$p= $p.chr(octdec(hexdec(substr($d,$k,6))) ^ ((($c ^ hexdec('AFE43')) ^ hexdec('399AA3')) ^ $e))  ;
}
return $p;
}

`
I have the decrypt function in PHP which is on my server.

I feel it is not a safe thing to store it in my server as if one day we get to be hacked. The hacker can encrypt easily our datas.

Please cooperate with me.