The shell script allows someone to access your operating system to list, create and remove files and directories as well as execute code on your server. how to prevent it?

?>


$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);
<?php
$pic = $_GET['pic'];
$taille = getimagesize("$pic");
$h_i = $taille[1];
$w_i = $taille[0];
if($h_i >$h_max)
{
$convert=$h_max/$h_i;
$h_i=$h_max;
$w_i=ceil($w_i*$convert);
}
if($w_i >$w_max)
{
$convert=$w_max/$w_i;
$w_i=$w_max;
$h_i=ceil($h_i*$convert);
};
$largeur = "$w_i";
$hauteur ="$h_i";
header("Content-Type: image/jpeg");
list($width, $height, $type, $attr) = getimagesize("$pic");
if($type == "1")
{
$img_in = imagecreatefromgif($pic);
}
if($type == "2")
{
$img_in = imagecreatefromjpeg($pic);
}
if($type == "3")
{
$img_in = imagecreatefrompng($pic);
}
$img_out = imagecreatetruecolor($largeur, $hauteur);
imagecopyresampled($img_out, $img_in, 0, 0, 0, 0, imagesx($img_out), imagesy($img_out), imagesx($img_in), imagesy($img_in));
$t = imagejpeg($img_out);
echo $t;
?>

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);

$output="i am NOoob....";
$newfile="ewanz.txt";
$file = fopen ($newfile, "w");
fwrite($file, $output);
fclose ($file);
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