$columns = ["sku","category","description","image","small_image","thumbnail", "price","color"];

$rootDir = ""; //Root directory where the image files are located
$file = fopen("filehandle.csv", "r"); //Open the old file for reading
$newFile = fopen("newfilehandle.csv", "w"); //Create a new file for writing

while (($data = fgetcsv($file)) !== FALSE) {
    $row = array_combine($columns, $data);
    $filename = "{$row['sku']}.jpeg";
    if (file_exists("$rootDir/$filename")) {
        $row['image'] = $filename;
        $row['small_image'] = $filename;
        $row['thumbnail'] =  $filename; 
    }
    fputcsv($newFile, array_values($row)); //Write data into new file
}

fclose($file);
fclose($newFile);