Terminate script when file reach certain size

I have a script which writes in the file in the directory where the script is. What I'm trying to do is to terminate/exit the program when the filesize reach 1MB.

What I have is

$total = 150;
$filename = "myFile.txt";

$fp = fopen($filename, 'a');
for ($i = 0; $i < $total; $i++) {
    $test = [];
    foreach ($characters as $field => $value) {
        $test[$field] = $value;
    }
    if (!fwrite($fp, implode($test))) {
        echo "Can't write to file: ($filename)";
        exit;
    }
}

What I tried is

$maxSize = 1000000; // 1 MB
$currentSize = 0; // 0 MB

$total = 150;
$filename = 'myFile.txt';

for ($i = 0; $i < $total; $i++) {
    $test = [];
    foreach ($characters as $field => $value) {
        $test[$field] = $value;
    }
    if ( $currentSize < $maxSize ) {
        if (!fwrite($fp, implode($test))) {
            echo "Can't write to file: ($filename)";

            $currentSize = filesize($filename);

            exit;
        }
    } else { 
        echo "Max size of file reached\n";
        exit;
    }
}

Basically I have added an if condition around the fwrite function and inside I have assign filesize to $currentSize which then check against $maxSize: $currentSize = filesize($filename);.

The script doesn't stop when the filesize is reached. I'm not sure why and if this is the right way of this.