Checking whether a file exists within a certain folder

I need to check whether a file exists using the is_dir() function. However I need to check whether a file exists within a certain folder. This is what I thought the code would look like:

if ($_GET['filename'])
	{
	
	$file = "courses/".$_GET['filename'];
	if (is_dir($file)){
	echo "yes";
	}else {
	echo "no";
	}
	}
	
	


else
{

die('must specify filename');

}

PHP:

this is supposed to work isn't it?
I'm getting no coding errors, the script is just not detecting the file. The funny thing is that when I just enter "courses" in the $file variable, it detects a folder name 'courses'
By the way, 'filename' is urlencoded $_GET from a previous script that lets the user choose the file to be displayed.
Is there a different way of doing this?

 

is_dir() checks specifically for directories. Use file_exists() or is_file() instead.

(Note that file_exists() works for directories AND files)

 

ah ok that makes sense...
thanks for the response