Tip when comparing variables

Alan @ CIT · 03-01-2008, 11:04 PM

Hi all,

Here is a little tip that I wish someone had told me when I started out with PHP

As I'm sure you all know, when comparing two values you would commonly use something like:

PHP Code:


			
if ($myVar == 'hello')

{

    // ...

}

However, if you are anything like me, you will occasionally type a single equals sign instead of double:

PHP Code:


			
if ($myVar = 'hello')

{

    // ...

}

Which introduces an anoying little hard-to-find bug in your code as instead of comparing the 2 values, it is actually setting $myVar to 'hello'!

Unfortunately, PHP sees this as perfectly valid so you don't get any error messages and aren't aware of any problems until it causes a bug in your application.

However, there is a little trick you can use to prevent this problem. When comparing values, use:

PHP Code:


			
if ('hello' == $myVar)

{

    // ...

}

As you can see, we have reversed the variables. This is still a perfectly valid if() statement but if you accidently type a single equals sign instead of two equals signs:

PHP Code:


			
if ('hello' = $myVar)

{

    // ...

}

PHP will throw you a nice error letting you know of the problem:

Code:

Parse error:  syntax error, unexpected '=' in ...

Once you get in to the habbit of writing your comparisons this way it will save you countless hours of debugging anoying little bugs (well, it has for me anyway

)

Alan

Orc · 03-01-2008, 11:37 PM

I've wondered whats the different between using this condition or a strcmp

Alan @ CIT · 03-01-2008, 11:42 PM

No real difference at all. Using strcmp would be slightly slower and doesn't work for numbers but other than that it works fine

Alan

Orc · 03-01-2008, 11:43 PM

Quote:

Originally Posted by Alan @ CIT

No real difference at all. Using strcmp would be slightly slower and doesn't work for numbers but other than that it works fine

Alan

I'd just go with the double equal sign operator.

sjaq · 03-02-2008, 12:15 PM

I always use three equal signs, which is a lot faster. And when I do a typo I still have two equal signs ;).

Shauny_B · 03-02-2008, 01:05 PM

Thanks for that Alan :)

vungom · 03-03-2008, 01:35 PM

i created 2 variables to compare but i think php only compares the first variable
something like this
if($a=='user'&& $b=='pwd')
{
echo "correct";
}

ReSpawN · 03-03-2008, 01:46 PM

PHP Code:


			
if ( ( $a == 'user' ) && ( $b == 'pwd' ) ) {
echo 'correct';
}

That is what it should be. The && can be referred to as "AND EQUAL" and || can be referred to as "OR". Same goes for the and - and or.

vungom · 03-03-2008, 01:56 PM

i also tried that one but still get the echo correct even if the pwd is not equal to variable b

vungom · 03-03-2008, 01:58 PM

btw i am comparing variable to variable from a text file something like this

if(($user==$username) && ($pwd==$pass) )
{
echo "correct"; }

thanks for the quick reply

ReSpawN · 03-03-2008, 04:08 PM

Post your entire script then.

PHP Code:


			
<?php
    
    $username = 'TalkPHP';
    $password = 'Wildhoney';
    
    $usd = 'TalkPHP';
    $pwd = 'Alan';
    
    if ( ($username == $usd) && ($password == $pwd) ) {
        echo 'Both values appear to be equal, so I return it true.';
    } elseif ( ($username == $usd) || ($password == $pwd) ) {
        echo 'Either username or password are not equal, but I return it true.';
    } elseif ( !($username == $usd) && !($password == $pwd) ) {
        echo 'I indicate that both are wrong, so I return it false.';
    }

Play around with the values a bit until you get the result you want. Otherwise, you should echo out all 4 variables with, for example, var_dump();. I hope this helps. Kinda finishing up Alan's topic here I guess.