how to inject variable into another variable of value

Question

$data = json_decode(file_get_contents("php://input"));
$Formula = $data->Formula; //getting value ($N-$D)*100.
$N = $data->Numerator; //getting value 100
$D = $data->Denominator; //getting value 20

I am getting JSON values like the above way. I want to inject $N,$D values to the variable of $Formula value. I want Output like below : (100-20)*100 = 8000

Could you please [edit] your question to add an example of the `$data`? — 0stone0, May 27 '21 at 15:22
However, this is very dangerous to do with user-supplied data, since it will execute any PHP functions. You really should find some other way to accomplish this that doesn't require evaluating user-supplied formulas. — Barmar, May 27 '21 at 15:26
i tried eval(). eval("\$Formula = \"$Formula\";"); it combines the values. but i need calculated value that is 8000 — konda, May 27 '21 at 15:49

0stone0 · Accepted Answer · 2021-05-27T16:18:03.317

First of all;

Caution

The eval() language construct is very dangerous because it allows execution of arbitrary PHP code.

Sharing my solution, using:

array_combine, array_map and array_keys to prefix all the keys with an $

^{Fastest way to add prefix to array keys?}
^{Keep in mind that this expects a formula were all the variables exist in $data}
strtr to replace $N with $data->N
eval() to calculate the string: calculate math expression from a string using eval

Code:

<?php

// Original data
$data = (Object) [
    'formula'   => '($N-$D)*100',
    'N'         => 100,
    'D'         => 20
];

// Create an object were 'N => 100' is saved as '$N => 100'
$ovars = get_object_vars($data);
$ovars = array_combine(array_map(function($k){ return '$' . $k; }, array_keys($ovars)), $ovars);

// Replace (oa) $N with $ovars['$N']
$sum = strtr($data->formula, $ovars);

// EVAL
$res = eval('return ' . $sum . ';');

// Show result
echo "SUM: $sum\nRESULT: $res";

Result:

SUM: (100-20)*100
RESULT: 8000

how to inject variable into another variable of value

1 Answers1