Using variables in if statements

Question

PHP novice here. I searched for this, but i'm sure i'm not using the right syntax regarding my issue. Apologies then if this is a duplicate:

I have these 3 variables:

$param = get_sub_field('custom_parameter');
$compare = get_sub_field('parameter_compare');
$param_val = get_sub_field('parameter_value');

each one would return this:

$param is 'my_parameter'
$compare is either '==', '<=', or '=>'
$param_val is something like '5' or any value that the user sets

What i have is an editing interface where the user can set their parameter name, set the compare and then add the value. To that they can also add an action that occurs if the parameter matches. I'm using this in conjunction with $_GET.

What i'd like to do is insert each variable from above into my if statement so the comparison is created by the user. However, it keeps giving me an error when i try to do this:

if($_GET[$param] $compare $param_val) {
// do something
}

The error i get is:

Parse error: syntax error, unexpected T_VARIABLE

This of course works just fine:

if($_GET[$param] == $param_val) {
// do something
}

Hopefully i've explained this well enough and any help is greatly appreciated.

Update: Thank you for answering this for me and jumping on it so quickly. Learned a ton here!!

Why do you need to do this? There's an answer, but it's best avoided — MDEV
– MDEV, Commented Jul 26, 2013 at 15:28
@tonoslfx That will just perform a boolean check on the string "my_parameter >= 5" rather than actually evaluate it — MDEV
– MDEV, Commented Jul 26, 2013 at 15:30
I have been programming PHP for well over 5 years now and I cannot imagine a situation where a dynamic operator is a good thing. You will have to elaborate. We can give you the answer but that would mean we hate you :) — MonkeyZeus
– MonkeyZeus, Commented Jul 26, 2013 at 15:30

Brad Christie · Accepted Answer · 2013-07-26 15:41:27Z

3

function comparer($param, $compare, $param_val)
{
  switch ($compare){
    case '==': return $param == $param_val;
    case '!=': return $param != $param_val;
    case '<=': return $param <= $param_val;
    case '>=': return $param >= $param_val;
    case '<':  return $param <  $param_val;
    case '>':  return $param >  $param_val;
    default: return FALSE;
  }
}

/* ... */

if (comparer($param, $compare, $param_val)){
  // true
}

Very simple method to get you going. I would, at all costs, resist the temptation to use eval, unless you want to invest a lot of time in sanitizing those three parameters.

Oh, and an example

edited Jul 26, 2013 at 15:41

answered Jul 26, 2013 at 15:31

Brad Christie

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2 Comments

mikevoermans Over a year ago

My answer was similar - but yours is cleaner - provided he doesn't need the if statement - perhaps just add in if (comparer($param, $compare, $param_val)) {...

Brad Christie Over a year ago

@RocketHazmat: except I don't mind having several exit points. ;p

mikevoermans · Accepted Answer · 2013-07-26 15:31:26Z

1

I think I would use a switch statement to avoid any scary eval code.

Such as:

switch($compare) {
    case '==':
        if($_GET[$param] == $param_val) {
            // do something
        }
    break;
    case '<=':
        if($_GET[$param] <= $param_val) {
            // do something
        }
    break;
    case '>=':
        if($_GET[$param] <= $param_val) {
            // do something
        }
    break;
}

answered Jul 26, 2013 at 15:31

mikevoermans

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1 Comment

blueplasma_dev Over a year ago

Thank you everyone for jumping on this and for teaching me about the security issues when using eval() with $_GET!!! This works perferctly. You guys are awesome!

Daniele Vrut · Accepted Answer · 2013-07-26 15:30:43Z

1

Look at eval()

http://php.net/manual/en/function.eval.php

With this you can parse a code you format in a string.

answered Jul 26, 2013 at 15:30

Daniele Vrut

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3 Comments

MDEV Over a year ago

Please don't suggest eval without explaining the security issues

gen_Eric Over a year ago

Using eval with $_GET is just asking for trouble. What if I send $_GET['val'] = "unlink('/var/www/index.php')"? ;-P

Daniele Vrut Over a year ago

of course. It's a novice and he must learn all, positive and negative things. I'll never use eval but it's in php lib :)

gen_Eric · Accepted Answer · 2013-07-26 15:31:46Z

1

The best way would be to make a function for this. Have that function uses a switch to determine the operator, then return the comparison.

function compare($a, $b, $operator){
    $ret = NULL;
    switch($operator){
        case '==':
            $ret = $a == $b;
            break;
        case '>=':
            $ret = $a >= $b;
            break;
        case '<=':
            $ret = $a <= $b;
            break;
    }
    return $ret
}

Then just simply call it:

if(compare($_GET[$param], $param_val, $compare)){
    // do something
}

answered Jul 26, 2013 at 15:31

gen_Eric

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Comments

martincarlin87 · Accepted Answer · 2013-07-26 15:35:37Z

Hmm interesting, I think I'd approach it like this (untested):

function comparison($param, $compare, $param_val) {
    if ($compare == '==') {
        if ($param == $param_val) {
            return true
        }
    }
    if ($compare == '<=') {
        if ($param <= $param_val) {
            return true
        }
    }
    if ($compare == '>=') {
        if ($param >= $param_val) {
            return true
        }
    }
}

Not very efficient or DRY, could probably use a switch as that would probably be better but this was the first thing to pop into my head.

Usage

if (comparison($param, $compare, $param_val)) {
    echo 'it's true';
} else {
    echo 'it's false';
}

Edit

As per usual, I have been beaten to the punch by better code :)

P.S. I'm not sure why you have $param and then use $_GET[$param] so I've just used $param in my answer.

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