How to increment unique filename based on directory array - php

Question

I'm creating an upload form that when uploading files will check to see if a file with the same name already exists in the directory. If a file with the same name is found, the script should increment the file by appending the file name with a number such as test1.txt test2.txt and so on until a match is not found. This is what I have come up with so far but wanted to see if there is a different approach I should take.
Also note, my filenames are already cleaned before they enter this part of the script so my filename and extension functions are simplified.
function filename($file){
return substr($file,0,strrpos($file,'.'));
}
function extension($file){
return strtolower(substr(strrchr($file,'.'),1));
}
$new_file = 'test.txt';
$dir = 'files';
$files = scandir($dir);
$exclude = array('.','..');
foreach($files as $file){
if(!in_array($file,$exclude)){
$file_array[] = $file;
}
}
$i = 1;
while(in_array($new_file,$file_array)){
$filename = filename($new_file);
$extension = extension($new_file);
if($i>1){
$num = strlen($filename);
$filename = substr($filename,0,-1);
$new_file = $filename.$i.'.'.$extension;
} else {
$new_file = $filename.$i.'.'.$extension;
}
echo $new_file.'<br>';
echo $i.'<br>';
$i++;
}
echo $new_file;