如何使用php从变量中获取String？

I have a variable post value and I would like to find it. For that I have a for loop and I'm trying to get the post data like that

$_POST['discount'.$i]

But when I echo it I get discount.$i instant of the value. What is wrong?

for ($i=0; $i<10;$i++){
    if(isset($_POST['discount'.$i]) && isset($_POST['percentage'.$i])){
        $discount[$i] = $_POST['discount'.$i];
        $percentage[$i] = $_POST['percentage'.$i];
        echo '$discount.$i';
        echo '$percentage.$i';
    }
}

Use like

for($i = 0; $i < 10; $i++)
{
    if(isset($_POST['discount' . $i]) && isset($_POST['percentage' . $i]))
    {
        $discount[$i] = $_POST['discount' . $i];
        $percentage[$i] = $_POST['percentage' . $i];
        echo $discount[$i]."<br>";
        echo $percentage[$i]."<br>";
    }
}

As i comment do this in two steps: $s = 'discount'.$i; echo $_POST[$s] and has a feedback of 3 votes, I make an answer, and got downvotes, I know the reason of downvotes, Now i make complete answer.

You need to make this two steps. First store in a variable and use the variable inside the $_POST.

for ($i=0; $i<10;$i++){
    $discounts = 'discount'.$i;
    $percentages = 'percentage'.$i;
    if(isset($_POST[$discounts]) && isset($_POST[$percentages])){
        $discount[$i] = $_POST[$discounts];
        $percentage[$i] = $_POST[$percentages];            
    }
}

Variables don't expand inside single quotes (echo '$discount.$i') and you need to echo the array value $discount[$i] not $discount.$i.
isset() allows empty values I would use !empty() instead, something like:

for ($i=0; $i<10;$i++){
    if(!empty($_POST["discount$i"]) && !empty($_POST["percentage$i"])){
        $discount[$i] = $_POST["discount$i"];
        $percentage[$i] = $_POST["percentage$i"];
        echo $discount[$i];
        echo $percentage[$i];
    }
}