Replace to skip how can i set

When upload same files or same name the script replace old file . How to skip new file if already exist.

// Check if form has been submitted
if(@$_POST['submit']){
ini_set("max_execution_time", 0);  // no time-outs!
ignore_user_abort(true);   // Continue downloading even after user closes the browser.
// URLS -- One on each line
$URL = $_POST['url'];
// Relative path to Save downloaded images
// Default is "downloads"
// Make sure that it is writable (chmoded correctly)
$folder = $_POST['folder'];
// Check if User has provided a local folder
if (!$folder || !isset($folder)){
// Generate error if left blank by user.
die ("Please specify local folder name");
}
// Split all URLS into an array
$urls = split("\n", $URL);
// Remove Carriage Returns (useful for Windows-based browsers)
$urls = str_replace("\r", "", $urls);
$mh = curl_multi_init();
foreach ($urls as $i => $url) {
$path = pathinfo($url);
$g=$folder . "/" . $path["basename"] ;
// Check if file already exists on local folder.
if(file_exists($g)){
// If exists, delete the file so it always contains the latest update.
unlink($g) or die("Unable to delete existing '$g'!");
}
// Update the user of what's going on
echo "$i) Downloading: from <b>$url</b> to <a href=\"$g\"><b>$g</b></a><br />";
if(!is_file($g)){
$conn[$i]=curl_init($url);
$fp[$i]=fopen ($g, "w");
curl_setopt ($conn[$i], CURLOPT_FILE, $fp[$i]);
curl_setopt ($conn[$i], CURLOPT_HEADER ,0);
// curl_setopt($conn[$i],CURLOPT_CONNECTTIMEOUT,1000);
curl_multi_add_handle ($mh,$conn[$i]);
}
}
do {
    if (curl_multi_select($mh, 0.25) == -1) {
        usleep(170);
    }

    $n = curl_multi_exec($mh, $active);
}
while ($active);
foreach ($urls as $i => $url) {
curl_multi_remove_handle($mh,$conn[$i]);
curl_close($conn[$i]);
fclose ($fp[$i]);
}
curl_multi_close($mh);
} // task closed

PHP:

 

http://php.net/manual/en/function.file-exists.php

You have this:

if(file_exists($g)){
// If exists, delete the file so it always contains the latest update.
unlink($g) or die("Unable to delete existing '$g'!");
}

Code (php):

So stop here instead of unlinking.