Stuff

<?php
$rowsAmt = 9;
$colsAmt = 9;

$boxGroupWidthSize = 3;
$boxGroupHeightSize = 3;

//a box is an indivdual cell
//a box group is a group of boxes


function boxIsInBoxGroup($boxRow, $boxCol) {
	$boxIsInBoxGroupRow = $boxRow % $boxGroupHeightSize;
	//if the $rowsAmt is 9 and $boxWidthSize is 3, $boxIsInBoxCol ranges
	//from 0 to 2, because that means there can be up to 3 distint box groups
	//a box groups is a group of boxes (aka cells) (3 by 3 cells for example) defined by
	
	$boxIsInBoxGroupCol = $boxCol % $boxGroupWidthSize;

	return array($boxIsInBoxGroupRow, $boxIsInBoxGroupCol);
}

function isStrInBoxGroup($something, $boxGroup) {
	//to get boxGroup array us boxIsInBoxGroup func

	
}
if (checkIfPost()) {

for ($c = 0; $c < $rowsAmt; $c ++;) {
	for ($c2 = 0; $c2 < $colsAmt; $c2 ++) {
		$dataByRow[$c][$c2] = $_POST['box'.$c.'_'.$c2];
		$dataByCol[$c2][$c] = $_POST['box'.$c.'_'.$c2];
	}
}

for ($num = 1; $num <= 9; $num ++) {
	for ($c = 0; $c < $rowsAmt; $c ++;) {
		
			if (!in_array($num, $dataByRow[$c]) && !in_array($num, $dataByCol)
				&& !inBoxGroup($c, 0)) {
			//if number c is not in the row and not in the column
			}

	}

}


echo '<form>
<table>';
for ($c = 0; $c < $rowsAmt; $c ++) {
	echo '<tr>';
		for ($c2 = 0; $c2 < $colsAmt; $c2 ++) {
			echo '<td>';
			echo '<input type="text" id="box'.$c.'_'.$c2.'">';
			echo '</td>';
		}
	echo '</tr>';
}
echo '</table>
</form>';
?>