PHP code runs fine by itself but will not work within a WordPress Page

Using a PHP in Pages plugin, I have been embedding my PHP files in WordPress Pages and calling them via short code.

The following code runs fine by itself. However, when I paste it into a WordPress page, it doesn’t do anything. All my other PHP files work fine.

Can anyone tell me what the possible problems are?

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<?php
session_start();

//handle form submission
if($_POST){
	//handle file upload

	// Edit upload location here
   	$destination_path = "/gallery/images/";

   	$rand_str = substr(str_shuffle(str_repeat('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789',5)),0,5);

   	$target_path = $destination_path . $rand_str . "_" . basename( $_FILES['image']['name']);

   	$filename = $rand_str . "_" . basename( $_FILES['image']['name']);

   	if (pathinfo($filename,PATHINFO_EXTENSION)!=='jpg' && pathinfo($filename,PATHINFO_EXTENSION)!=='jpeg' && pathinfo($filename,PATHINFO_EXTENSION)!=='gif' && pathinfo($filename,PATHINFO_EXTENSION)!=='png' && pathinfo($filename,PATHINFO_EXTENSION)!=='JPG' && pathinfo($filename,PATHINFO_EXTENSION)!=='JPEG' && pathinfo($filename,PATHINFO_EXTENSION)!=='GIF' && pathinfo($filename,PATHINFO_EXTENSION)!=='PNG') {
		echo "Not valid image file.";
   	}
   	elseif(@move_uploaded_file($_FILES['image']['tmp_name'], $target_path)) {

      	echo "File uploaded and added to selected categories.";
      	//which categories are we adding the file to?
   	$cats="";
   	$result=mysql_query("select * from categories where parent_category='2' ") or die(mysql_error());
   	while($row=mysql_fetch_assoc($result)){
   		$cats .= $row['id'].", ";
   	}

   	//do we also add it to categories under parent category 1?
   	if($_POST['cat1too']==1){
   		$result=mysql_query("select * from categories where parent_category='1' ") or die(mysql_error());
   		while($row=mysql_fetch_assoc($result)){
   			$cats .= $row['id'].", ";
   		}
   	}

   	mysql_query("insert into object_dir (content,category) values ('$target_path','$cats') ") or die(mysql_error());

   	}

   	// Close Session

mysql_close();  

}

?>

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Thanks,

—
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