<?php
// your code goes here
$length=10;
?>

<script>
	function syncCheckbox(obj1, obj2){
		var checkboxA = document.getElementById(obj1);
		var checkboxB = document.getElementById(obj2);
		checkboxB.checked = checkboxA.checked
	}
</script>
<form>
<?php
	$i=0;
	while($i++ < $length){
		echo <<<EOT
		<input type='checkbox' id="a{$i}" onclick="syncCheckbox('a{$i}','b{$i}')"></input>
		<input type='checkbox' id="b{$i}"></input>
		<br>

EOT;
	}
?>
</form>