How to parse a string of boolean logic in PHP

Question

I\'m building a PHP class with a private member function that returns a string value such as:

\'true && true || false\'

to a public

Answer 1

Let's assume eval() is an ok/good solution in your case.

class Foo {
  private function trustworthy() {
    return 'true && true || false';
  }

  public function bar() {
    return eval('return '.$this->trustworthy().';');
  }
}

$foo = new Foo;
$r = $foo->bar();
var_dump($r);

prints bool(true)

Answer 2

eval() will work perfectly fine for this, but remember you have to tell it to return something.

$string = "true && true || false";
$result = eval("return (".$string.");"); // $result will be true

Also make sure you sanitize any user inputs if putting them directly into an eval().

Answer 3

Just stumbled upon this question, but being fairly uneasy about using eval, I decided to keep looking for a better solution.

What I discovered is yet another wonderful use for PHP's filter_var function, when passing in the FILTER_VALIDATE_BOOLEAN flag (of which there are many).

This "one line" function seems to do well at safely converting a string (or other) object to a boolean:

<?php

/**
 * Uses PHP's `filter_var` to validate an object as boolean
 * @param string $obj The object to validate
 * @return boolean
 */
function parse_boolean($obj) {
    return filter_var($obj, FILTER_VALIDATE_BOOLEAN);
}

And, a little testing:

/**
 * Let's do some testing!
 */
$tests = array (
    "yes",
    "no",
    "true",
    "false",
    "0",
    "1"
);

foreach($tests as $test) {

    $bool = parse_boolean($test);

    echo "TESTED: ";
    var_dump($test); 

    echo "GOT: ";
    var_dump($bool);

    echo "\n\n";

}

Output:

/*
TESTED: string(3) "yes"
GOT: bool(true)


TESTED: string(2) "no"
GOT: bool(false)


TESTED: string(4) "true"
GOT: bool(true)


TESTED: string(5) "false"
GOT: bool(false)


TESTED: string(1) "0"
GOT: bool(false)


TESTED: string(1) "1"
GOT: bool(true)
*/

I haven't looked deep enough, but it's possible that this solution relies on eval down the line somewhere, however I'd still side with using those over plain evaling since I assume that filter_var would also handle sanitizing any input before piping it through eval.