PHP values from input keep resetting after enter - php

Question

I am trying to create a program that prints off an array of five user-inputted elements. Each input box should take in a value from the user, store it in an array, and print off that array value. However, the values reset and become null when you enter text into the next input box. You can see what I mean by looking at the site.
https://people.emich.edu/ghaines1/cosc231/stringReverse.php
I eventually want to print off the values of the strings in reverse as well. I'm sure that's quite easy but I haven't looked into it yet, as I need this first part to work first.
So ideally, the PHP portion should print off:
$input[0]
$input[1]
$input[2]
$input[3]
$input[4]
assuming that it has retained those values. Again, my issue is that I cannot print more than one value because they reset and become null when I enter a value into the next text box. Here's the code:
<form action="stringReverse.php" method = "post">
<input name="st1" type="text" size="50" placeholder="Enter phrase"><br><br>
</form>
<form action="stringReverse.php" method = "post">
<input name="st2" type="text" size="50" placeholder="Enter phrase"><br><br>
</form>
<form action="stringReverse.php" method = "post">
<input name="st3" type="text" size="50" placeholder="Enter phrase"><br><br>
</form>
<form action="stringReverse.php" method = "post">
<input name="st4" type="text" size="50" placeholder="Enter phrase"><br><br>
</form>
<form action="stringReverse.php" method = "post">
<input name="st5" type="text" size="50" placeholder="Enter phrase"><br><br>
</form>
<?php
if ($_SERVER["REQUEST_METHOD"]=="POST") {
$input = array (0, 0, 0, 0, 0);
if (is_null($_POST["st1"]) == false) {
$input[0] = $_POST["st1"];
echo $input[0] . "<br>";
}
if (is_null($_POST["st2"]) == false) {
$input[1] = $_POST["st2"];
echo $input[1] . "<br>";
}
if (is_null($_POST["st3"]) == false) {
$input[2] = $_POST["st3"];
echo $input[2] . "<br>";
}
if (is_null($_POST["st4"]) == false) {
$input[3] = $_POST["st4"];
echo $input[3] . "<br>";
}
if (is_null($_POST["st5"]) == false) {
$input[4] = $_POST["st5"];
echo $input[4] . "<br>";
}
}
?>

Answer 1

its because you are defining a form for each input
change html to:
<form action="stringReverse.php" method = "post">
<input name="st1" type="text" size="50" placeholder="Enter phrase"><br><br>
<input name="st2" type="text" size="50" placeholder="Enter phrase"><br><br>
<input name="st3" type="text" size="50" placeholder="Enter phrase"><br><br>
<input name="st4" type="text" size="50" placeholder="Enter phrase"><br><br>
<input name="st5" type="text" size="50" placeholder="Enter phrase"><br><br>
</form>