Increment the scores in different text files

If I want to store and increment the score of the players in a text file. Every round generates different text file. Here is the code:

$string = '';
for ($i = 0; $i < 20; $i++) {
    $d = rand(1, 30) % 2;
    $char = $d ? chr(rand(65, 90)) : chr(rand(48, 57));
    $string .= $char;
}
$ourFileName = $string . ".txt";

$counter = file($ourFileName);

echo $counter;
if (strcasecmp($answer3, $answer4) == 0) {
    $counter[0]++; // increase the counter by one
    echo "<h3 style='color:White; margin-left:600px;'>  Player 2 score: &nbsp; $counter[0] </h3>";
    echo " <h3 style='color:White; margin-left:600px;'>Well done, the correct answer is :  $answer3 </h3>";
} else {
    $counter[0]--; // increase the counter by one
    echo "<h3 style='color:White; margin-left:600px;'> Player 2 score: &nbsp; $counter[0]   </h3>";
    echo "<h3 style='color:White; margin-left:600px;'> The correct answer is :开发者_如何学Go $answer3 </h3>";
}
$fp = fopen($ourFileName, "w");
fwrite($fp, $counter[0]);
fclose($fp);

this code generates a file with random string but it doesn't increment the score and renders null. If I create a file with a specific name it works but I want the files to be with different names. Is there any problem in my code???