Optimizing PHP script fetching entire HTML pages

The following script should get links which are stored in a text file (line by line), then put them into an array and finally scan each links' source code for a certain line. If this line is found, it will be pasted into a CSV file.

It works fine so far, but it takes ages to finish, since each link is 'opened' and the complete source code for that link is scanned for this specific line.

I'm looking for ideas on how to optimize the code to run faster.

Here is my code:

$filename = "products.txt";
$writecsv = "notavailable.csv";
global $products;

$ch = curl_init();
curl_setopt($ch, CURLOPT_COOKIEJAR, "/tmp/abCk.txt");
curl_setopt($ch, CURLOPT_URL,"https://www.websitegoeshere.com");
curl_setopt($ch, CURLOPT_POST, 1);
curl_setopt($ch, CURLOPT_POSTFIELDS, "Login=USERNAME&Password=PASSWORD");

ob_start();      // prevent any output
curl_exec ($ch); // execute the curl command
ob_end_clean();  // stop preventing output

curl_close ($ch);
unset($ch);

$ch = curl_init();
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch, CURLOPT_COOKIEFILE, "/tmp/abCk.txt");

// Open the file
$fp = @fopen($filename, 'r') or die("products.txt not found"); 

// Add each line to an array
if ($fp) {
   $products = explode("\n", fread($fp, filesize($filename)));
}

fclose($fp);

$fpcsv = fopen($writecsv, 'w') or die("notavailable.csv not found");

foreach($products as $key => $val) {
    curl_setopt($ch, CURLOPT_URL,$val);
    $buf2 = curl_exec ($ch);
    $html = htmlentities($buf2);
    if (strpos($html, "/extension/silver.project/design/sc_base/images/available_yes.gif") !== false) {
        fputcsv($fpcsv, "available");
    } else {
        fputcsv($fpcsv, "not available");
    }
}

fclose($fpcsv);


curl_close ($ch);
echo "csv written successfully."

Any help is really welcomed. Thanks in advance!