PHP Variable undefined for eval function - php

Question

I'm new to PHP and coding in general.
Currently I'm trying to make a Simple Calculator.
For this I have the two scripts calculator.php and array.php
My Idea is to store all the inputs the user gives in an array so I can calculate them using the eval() function. Because I need a string for this I converted the array using a foreach loop.
But when I try to use the variable outside the foreach loop I get the following errors:
Notice: Array to string conversion on line 17
Array
Parse error: syntax error, unexpected ';', expecting '(' in /Applications/XAMPP/xamppfiles/htdocs/dashboard/array.php(22) : eval()'d code on line 1
What can I do to fix this problem?
array.php
<?php
session_start();
if (!is_array($_SESSION)) {
$_SESSION['persistentValues'] = array();
}
if (isset($_POST['button'])) {
$_SESSION['persistentValues'][] = $_POST['button'];
}
foreach ($_SESSION as $value) {
echo $value;
}
if (isset($_POST["result"])) {
$p = eval($value.";");
echo $p;
}
?>
calculator.php
<?php
include ('./array.php');
?>
<div style="padding-left:40px">
<h2> PHP Calculator</h2>
<form method="post" action="">
Enter value: <input type="text" name="value"> <br> <br>
<div style="padding-left: 105px">
<input type="submit" value="9" name="button">
<input type="submit" value="8" name="button">
<input type="submit" value="7" name="button">
<input type="submit" value="+" name="button">
<br>
<input type="submit" value="6" name="button">
<input type="submit" value="5" name="button">
<input type="submit" value="4" name="button">
<input type="submit" value="-" name="button" style="padding-left: 9px">
<br>
<input type="submit" value="3" name="button">
<input type="submit" value="2" name="button">
<input type="submit" value="1" name="button">
<input type="submit" value="/" name="button" style="padding-left: 9px">
<br>
<input type="submit" value="0" name="button" style="padding-left:33px">
<input type="submit" value="." name="button" style="padding-right:9px">
<input type="submit" value="x" name="button" style="padding-left: 7px">
<br>
</div>
<br>
<input type="submit" value="Calculate" name="result">
</form>
</div>

Answer 1

You're repeatedly confusing the whole $_SESSION variable with $_SESSION['persistentValues']. The latter is where you're holding the buttons as you submit them.
The value of $value is an array because you're looping over $_SESSION to set it, and the value of the session variable is an array. The argument to eval() is a string, and when you convert an array to a string it becomes the string Array, which isn't an expression that can be evaluated.
You can use implode() to concatenate all the elements of an array into a string. So if the session array contains ['9', '3', '+', '8'] it will set $value to '93+8', which you can then pass to eval().
The argument to eval() needs to be a statement. So you should put the echo command in the argument.
<?php
session_start();
if (!is_array($_SESSION['persistentValues'])) {
$_SESSION['persistentValues'] = array();
}
if (isset($_POST['button'])) {
$_SESSION['persistentValues'][] = $_POST['button'];
}
$value = implode('', $_SESSION['persistentValues']);
echo $value;
if (isset($_POST["result"])) {
eval("echo $value;");
}
?>