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I want to know what are the main different between 3 tier and 2 tier architecture And also what are the advantages and disadvantages each one

i was trying to have a code from which i can upload the files on server via php and i got this code below from w3school but its not working can u help me with thisthanks_________________________________________________________ <?php$allowedExts = array("jpg", "jpeg", "gif", "png");$extension = end(explode(".", $_FILES["file"]["name"]));if ((($_FILES["file"]["type"] == "image/gif")|| ($_FILES["file"]["type"] == "image/jpeg")|| ($_FILES["file"]["type"] == "image/pjpeg"))&& ($_FILES["file"]["size"] < 20000))&& in_array($extension, $allowedExts)){if ($_FILES["file"]["error"] > 0){echo "Return Code: " . $_FILES["file"]["error"] . "<br />";}else{echo "Upload: " . $_FILES["file"]["name"] . "<br />";echo "Type: " . $_FILES["file"]["type"] . "<br />";echo "Size: " . ($_FILES["file"]["size"] / 1024) . " Kb<br />";echo "Temp file: " . $_FILES["file"]["tmp_name"] . "<br />";if (file_exists("upload/" . $_FILES["file"]["name"])){echo $_FILES["file"]["name"] . " already exists. ";}else{move_uploaded_file($_FILES["file"]["tmp_name"],"upload/" . $_FILES["file"]["name"]);echo "Stored in: " . "upload/" . $_FILES["file"]["name"];}}}else{echo "Invalid file";}?>

How do I get php to put the contents, and only the contents, of a file being uploaded into an array?