Checking the Length of Strings and Arrays in PHP

Let we have a variable $str, in which some arbitrary string is stored:

<?php
	$str = '12345';
?>

Let's write a condition that will display a message on the screen if the length of the string is equal to or greater than three characters:

<?php
	$str = '12345';
	
	if (strlen($str) >= 3) {
		echo '!';
	}
?>

The variable $arr contains some array with numbers. Write a condition that will check that the array has 3 elements. If so, display the sum of the array's elements on the screen.