TalkPHP - Tip when comparing variables

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Tip when comparing variables

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 :-D)

Alan

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

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

Quote:

Originally Posted by Alan @ CIT (Post 11798)

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.

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

Thanks for that Alan :)

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";
}

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.

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

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

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. :-P