visto che il forum è condivisione, vi do la soluzione che ho trovato:
spero a qualcuno torni utile...
Codice PHP:
function checkEdit($fileIn$fileUp){
    
// FILE IN: il file già presente nel database
    // FILE UP: il file che viene modificato
    
$tmp="tmp/";
    
$dir=opendir($tmp);
    if(!
is_dir($tmp)) mkdir($tmp);
    if(
move_uploaded_file($fileUp['tmp_name'],$tmp.$fileUp['name'])){
        print 
"File caricato!";
        
// procedo con il raffronto
        
while (false !== ($filereaddir($dir))){
            if(
$file==$fileUp['name']){
                
$file1=fread(fopen($tmp.$file,"rb"),filesize($tmp.$fileUp['name']));
                break;
            }
        }
        
$file2=fread(fopen($fileIn,"rb"),filesize($fileIn));
        if(
$file1==$file2){
            print 
"IL FILE E' GIA PRESENTE";
            if(
unlink($tmp.$fileUp['name']))
                return 
true;
            }else{
                return 
false;
            }
        }else{
            print 
"MUOVO IL FILE";
            if(
unlink($fileIn)){
                if(
copy($tmp.$fileUp['name'],$fileIn)){
                    if(
unlink($tmp.$fileUp['name'])){
                        return 
true;
                    }else{
                        return 
false;
                    }
                }else{
                    return 
false;
                }
            }else{
                return 
false;
            }
        }
    }else{
        return 
false;
    }