PHP practice function ($result) can't be called - php

Question

I am doing PHP practice and I am stuck on this part.
My problem is when I do simple math like 1+1 or 1/1 etc. The $result won't display. I am confused about which part of my code is causing the problem. Please enlighten me on my mistakes
Thank you.
<?php
$first_num = $_POST['first_num'];
$second_num = $_POST['second_num'];
$operator = $_POST['operator'];
$result = '';
function cal_result($first_num,$second_num,$operator){
if (is_numeric($first_num) && is_numeric($second_num)) {
switch ($operator) {
case "Add":
$result = $first_num + $second_num;
break;
case "Subtract":
$result = $first_num - $second_num;
break;
case "Multiply":
$result = $first_num * $second_num;
break;
case "Divide":
$result = $first_num / $second_num;
}
return $result;
}
}
?>
<form action="simple_calculator.php" method="post" id="quiz-form">
<p>
<input type="number" name="first_num" id="first_num" required="required" value="<?php cal_result($first_num,$second_num,$operator); ?>" /> <b>First Number</b>
</p>
<p>
<input type="number" name="second_num" id="second_num" required="required" value="<?php cal_result($first_num,$second_num,$operator); ?>" /> <b>Second Number</b>
</p>
<p>
<input readonly="readonly" name="result" value="<?php echo $result; ?>"> <b>Result</b>
</p>
<input type="submit" name="operator" value="Add" />
<input type="submit" name="operator" value="Subtract" />
<input type="submit" name="operator" value="Multiply" />
<input type="submit" name="operator" value="Divide" />
</form>

Answer 1

I think you are confused by the short open tag
if I print
<?=$result;?>
or
<?=cal_result($first_num,$second_num,$operator);?>
this is the same thing like
<?php echo $result;?>
or
<?php echo cal_result($first_num,$second_num,$operator);?>
If you call a function that ´returns´ values, and you want to use them , you must also print them out...
also check out global variables. If you want to use $result outside the function scope you must declare it a global variable.
<?php
$result = '';
$first_num = 0;
$second_num = 0;
$ops = array( "+",
"-",
"×", // HTML : × , ASCII CODE : 158
"÷"); // HTML : ÷ , ASCII CODE : 246
// checking if the first number is what he should be
if ( isset($_POST['first_num']) &&
!empty($_POST['first_num']) &&
is_numeric($_POST['first_num']) )
{
$first_num = $_POST['first_num'];
}
// checking if the second number is what he should be
if ( isset($_POST['second_num']) &&
!empty($_POST['second_num']) &&
is_numeric($_POST['second_num']) )
{
$second_num = $_POST['first_num'];
}
function cal_result($first_num,$second_num,$operator){
// optionally you can assign global
global $result;
// here we prevent injection by acceptint only operators from the ops array
if (isset($_GET['operator']) && in_array($_GET['operator'],$ops)) {
switch ($_GET['operator']) {
case $ops[0]: // first element of array ops is Add
$result = $first_num + $second_num;
// remember this function is just returning the $result
// variable it is not printing it, you will have to assiging
// the function to a variable if you want to use the
return $result;
break;
case $ops[1]: // second element of array ops is Subtract
$result = $first_num - $second_num;
return $result;
break;
case $ops[2]: // third element of array ops is Multiply
$result = $first_num * $second_num;
return $result;
break;
case $ops[3]: // fourth element of array ops is Divide
// divide by zero problem
if($second_num !== 0){
$result = $first_num / $second_num;
}else{
$result = "Err: Div By Zero";
}
return $result;
break;
}
}
}
}
?>
<p>
<!--// now you have 2 possibilities : //-->
<!--// first you can call the global variable $result //-->
<label for="quiz-form"><?=$result;?></label>
<!--// the second possibility is to call the function //-->
<label for="quiz-form"><?=cal_result($first_num,$second_num,$operator);?></label>
<b>Result</b>
</p>
<!-- for post you should also set enctype for security reasons -->
<form action="simple_calculator.php"
method="post"
id="quiz-form"
enctype="application/x-www-form-urlencoded">
<p>
<input type="number"
name="first_num"
id="first_num"
required="required"
value="<?=$first_num;?>" />
<b>First Number</b>
</p>
<p>
<input type="number"
name="second_num"
id="second_num"
required="required"
value="<?=$second_num;?>" />
<b>Second Number</b>
</p>
<input type="submit" name="operator" value="+" alt="Add" />
<input type="submit" name="operator" value="-" alt="Subtract" />
<input type="submit" name="operator" value="×" alt="Multiply" />
<input type="submit" name="operator" value="÷" alt="Divide" />
</form>