Using PHP function for shortening repeating code - php

Question

My code in PHP is pretty long and I want to make it shorter with creating one function with different values and than I would just write one line with function name instead of many lines of code, but it doesn't seem to work.
This is that repeating code:
if (!isset($_POST['ID_user']) || empty($_POST['ID_user'])) {
$_SESSION['ID_user_missing'] = "error";
header("location: index.php");
} else {
$ID_user = $_POST['ID_user'];
}
if (!isset($_POST['meta_name']) || empty($_POST['meta_name'])) {
$_SESSION['meta_name_missing'] = "error";
header("location: index.php");
} else {
$meta_name = $_POST['ID_user'];
}
if (!isset($_POST['meta_value']) || empty($_POST['meta_value'])) {
$_SESSION['meta_value_missing'] = "error";
header("location: index.php");
} else {
$meta_value = $_POST['meta_value'];
}
And this was the plan, instead of that code up ther, I would just have this down below:
function ifIssetPost($value) {
if (!isset($_POST[$value]) || empty($_POST[$value])) {
$_SESSION[$value.'_chybi'] = "error";
header("location: index.php");
} else {
$$value = $_POST[$value];
}
}
ifIssetPost('ID_user');
ifIssetPost('meta_name');
ifIssetPost('meta_value');
But it just doesn't work, when you try to echo for example variable $meta_name it shows that it's empty. Can you help me ? Thank you very much.
NOTE: when I doesn't that function and do it the long way, everything works just fine, but the problem comes when I use that function.

Answer 1

The variable is in the scope of function. That's why you cannot access to it outside the function. You could return the value:
function ifIssetPost($value) {
if (empty($_POST[$value])) { // Only empty is needed (as pointed out by #AbraCadaver)
$_SESSION[$value.'_chybi'] = "error";
header("location: index.php");
exit; // add exit to stop the execution of the script.
}
return $_POST[$value]; // return value
}
$ID_user = ifIssetPost('ID_user');
$meta_name = ifIssetPost('meta_name');
$meta_value = ifIssetPost('meta_value');

Answer 2

You can also follow your specification, using $$value:
function ifIssetPost($value) {
if (!isset($_POST[$value]) || empty($_POST[$value])) {
$_SESSION[$value.'_chybi'] = "error";
header("location: index.php");
} else {
return $_POST[$value];
}
}
$value = 'ID_user';
$$value = ifIssetPost($value);
echo $ID_user;
$value = 'meta_name';
$$value = ifIssetPost($value);
echo $meta_name;

Answer 3

You can use an array to iterate over the $_POST vars. If you want to declare a variable using a string or another variable containing an string, you need to use {}. like ${$value}
$postValues = ["ID_user", "meta_name", "meta_value"];
foreach ($postValues as $value) {
if (!isset($_POST[$value]) || empty($_POST[$value])) {
$_SESSION[$value."_missing"] = "error";
header("location: index.php");
} else {
${$value} = $_POST[$value];
}
}