Here is my answer:
<?php
$number1 = intval(readline());
$number2 = intval(readline());
$number3 = intval(readline());
$endResult = addAndSubtract($number1, $number2, $number3);
echo $endResult;
function addAndSubtract($n1, $n2, $n3) {
$result = 0;
$result += $n1 + $n2;
$result -= $n3;
return $result;
}