When ProgressBar has been displayed, cleared, and then displayed again, the second display() call thinks the progress bar is still visible, and incorrectly rewinds the cursor up to previous lines when the format string has multiple lines.

This test program:

echo "1\n2\n3\n4\n5\n";
sleep(1);

$progressBar = new ProgressBar($output, 100);
$progressBar->setFormat("hello\nworld\n");
$progressBar->display();
sleep(1);

$progressBar->clear();
sleep(1);

$progressBar->display();
sleep(1);

results in output where 4 and 5 have been overwritten:

1
2
3
hello
world

Perhaps clear() should set $this->firstRun back to true. (Or to be more flexible in case interpolated values contain newlines, overwrite() could track how many lines were actually written.)