The longest period iterating quine
PHP, period 2,100,000,000

Who would've thought that this is possible in PHP?! :-)

This is actually my first ever quine and it's 99 bytes long:

&lt;?$i=1;$i*=21e8&gt;$i;printf($a='&lt;?$i=%d;$i*=21e8&gt;$i;printf($a=%c%s%c,++$i,39,$a,39);',++$i,39,$a,39);


Although PHP supports bigger numbers than 2 * 10^8 by switching from integer to double, the increment no longer works (leads to an infinite loop) and I haven't found another solution which fits into the 100 bytes. Yet.

Proof is fairly simple, since it's just counting up on each iteration until it reaches the reset point at 2.1 billion.

Credits to dave, who posted the approach in pseudo code in the comments and to Bob Twells, from whom I copied the code for a minimal PHP quine.

Test program (sloooooow):

&lt;?php
$o = file_get_contents('quine.php');
for ($i = 0; $i &lt; 22e8; $i++) {
    if ($i%2==0) exec('php q &gt; p'); else exec('php p &gt; q');
    $a = file_get_contents(($i%2==0) ? 'p' : 'q');
    echo "\r" . str_pad($i,6,' ') . ":\t$a";
    if ($a == $o) {
        die;
    }
}


Well, at least I'm the first one to answer.