Elseif Construct in PHP

The elseif construct allows you to specify conditions in the else block. Let's look at an example:

<?php
	$num = 1;
	
	if ($num === 1) {
		echo 'variant 1';
	} elseif ($num === 2) {
		echo 'variant 2';
	} elseif ($num === 3) {
		echo 'variant 3';
	}
?>

The advantage of using elseif instead of multiple if statements is the ability to catch the situation when the value of the variable $num does not match any of the conditions:

<?php
	$num = 1;
	
	if ($num === 1) {
		echo 'variant 1';
	} elseif ($num === 2) {
		echo 'variant 2';
	} elseif ($num === 3) {
		echo 'variant 3';
	} else {
		echo 'variant not supported';
	}
?>

The variable $day contains some number from the interval 1 to 31. Determine into which decade of the month this number falls (the first, second, or third).

Modify the previous task so that, if the variable $day contains not a number from 1 to 31, an error message is displayed.