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| Comparison operators: When equals doesn't equal equals [message #1140] |
Fri, 06 April 2007 10:22  |
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 KathyReid
Messages: 224
Registered: October 2006
Location: Geelong, Victoria, Austra... |
Member |
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This article provides a brief tutorial on a common topic of confusion amongst PHP developers - the seemingly similar operators; equals (==) and identical (===).
During your PHP programming, you will have undoubtedly used the equals operator to perform value comparisons, usually to do conditional processing. For instance;
<?php
$myVar1 = 'cat'; // a string
$myVar2 = 'dog'; // also a string
$myVar3 = 'cat'; // again a string
// $myVar1 == $myVar2 will evaluate to boolean false
// $myVar1 == $myVar3 will evaluate to boolean true
if ($myVar1 == $myVar2)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
if ($myVar1 == $myVar3)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
$myVar4 = '42'; // a string
$myVar5 = 42; // an integer
// $myVar4 == $myVar5 will evaluate to boolean true
// because == does a value comparison only
if ($myVar4 == $myVar5)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
$myVar6 = 42.00; // a float
$myVar7 = 42; // an integer
// $myVar6 == $myVar7 will evaluate to boolean true
// because == does a value comparison only
if ($myVar6 == $myVar6)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
?>
The identical operator goes one step further, and compares not just the value, but the type of the operands.
Let's use the identical operator on the examples given above and see what happens;
<?php
$myVar1 = 'cat'; // a string
$myVar2 = 'dog'; // also a string
$myVar3 = 'cat'; // again a string
// $myVar1 === $myVar2 will evaluate to boolean false
// $myVar1 === $myVar3 will evaluate to boolean true as both type and value are identical
if ($myVar1 === $myVar2)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
if ($myVar1 === $myVar3)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
$myVar4 = '42'; // a string
$myVar5 = 42; // an integer
// $myVar4 === $myVar5 will evaluate to boolean false
// because the types are different - one is a string, the other an integer
if ($myVar4 === $myVar5)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
$myVar6 = 42.00; // a float
$myVar7 = 42; // an integer
// $myVar6 == $myVar7 will evaluate to boolean false
// because the types are different - one is a float, the other an integer
if ($myVar6 === $myVar7)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
?>
The identical operator is also useful for comparing arrays;
<?php
$myVar8 = Array('blue'); // an array with one element
$myVar9 = 'blue'; // a string
// $myVar8 === $myVar9 will evaluate to boolean false
// because one is a string, and the other is an array of one element (even though the one element is a string)
if ($myVar8 === $myVar9)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
$myVar10 = Array(1,1,3,5,8,13,21,35); // an array of integers
$myVar11 = Array(1.0, 1.0, 3.0, 5.0, 8.0, 13.0, 21.0, 35.0); // an array of floats
$myVar12 = Array('1', '1', '3', '5', '8', '13', '21', '35'); // an array of strings
// even though the variables above are all arrays,
// they are arrays of different types of values (integer, float, string)
// and are therefore not identical
if ($myVar10 === $myVar11)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
if ($myVar10 === $myVar12)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
if ($myVar11 === $myVar12)
{
echo 'TRUE';
} else
{
echo 'FALSE';
}
?>
In many cases, the equals operator will be sufficient for the task at hand. However, there are many tasks for which the identical operator is superior;
- When comparing variables that will be used to update a database (for instance, ensuring that an expected integer value is in fact an integer instead of a string designed to inject malicious SQL)
- When comparing arrays where elements of the same type (say floats, integers, strings etc) are assumed. Remember, assuming that values passed are those that are expected is a fundamental mistake made by many programmers.
- Comparing that resources (such as database connections, file handlers etc) are of the expected type.
More information on the equals and identical operator can be found at;
http://au2.php.net/operators.comparison
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| Re: Comparison operators: When equals doesn't equal equals [message #1394 is a reply to message #1140 ] |
Thu, 12 July 2007 16:36  |
elfsie
Messages: 4
Registered: July 2007
Location: Los Angeles, CA |
Shiny and New |
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Hi Kathy & everyone,
I'm new to the site and love it. Thanks for all your hard work and great insights.
About the == versus === issue: I have a session variable that can be unset or set to 0, 1 or "error". Here's an if/else test that sets its value:
//if not yet set, set to 0
if (!isset($_SESSION[$var])){
$_SESSION[$var] = 0;
}
//if set to error (which happens elsewhere), reset to 0
elseif ((isset($_SESSION[$var])) && ($_SESSION[$var]=="error")){
$_SESSION[$var] = 0;
}
else {
$_SESSION[$var] = 1;
}
In the elseif test above, if $_SESSION[$var] is 0, it will pass that test and be set to 0. If I change the == to ===, it then fails the test as it should. But for my future reference and edification, how does 0 sort of equal "error"? Is the string "error" a reserved word, or does it behave like the word false behaves as a boolean? I couldn't find anything in the documentation.
(BTW, how do I keep the indents in my code? They are being stripped out)
Thanks!
Erica C.
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| Re: Comparison operators: When equals doesn't equal equals [message #1402 is a reply to message #1140 ] |
Sun, 15 July 2007 00:05 |
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KathyReid
Messages: 224
Registered: October 2006
Location: Geelong, Victoria, Austra... |
Member |
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Hi Elfsie,
Here's a useful approach for investigating this one;
First of all I will make the following assumptions;
- 'error' is a string literal and has not been defined as a constant. If it is a constant, it should be written as ERROR to fit with industry standards (constants should be referenced as upper case - it makes it easy to know that they are constants
 )
- $_SESSION["$var"] is defined as a string ('error' is a string literal)
- 'error' is not a reserved word according to the reserved words list in the manual http://au3.php.net/manual/en/reserved.php
Next, let's break down the PHP and step it out in plain English;
<?php
//if not yet set, set to 0
if (!isset($_SESSION[$var])){
$_SESSION[$var] = 0;
}
?>
If the session variable $var is not set, we will (implictly) set it and (explictly) assign it the value of zero. Zero is an integer in this case as it is not encased in quotes ie '0'.
<?php
//if set to error (which happens elsewhere), reset to 0
elseif ((isset($_SESSION[$var])) && ($_SESSION[$var]=="error")){
$_SESSION[$var] = 0;
}
?>
If the session variable $var is set and it is equal in value (not in type) to the string literal "error" then set $var to the integer value of zero.
<?php
else {
$_SESSION[$var] = 1;
}
?>
Else set the session variable to the integer value of 1.
So, what does this all mean in terms of comparison operators?
Let's concentrate on this line;
<?php
elseif ((isset($_SESSION[$var])) && ($_SESSION[$var]=="error")){
?>
First of all, "error" is in double quotes. PHP will attempt to evaluate anything in between double quotes and will check here to see if there are any statements for evaluation (see the post about single vs double quotes). Here, it should be written as 'error' because it is being treated as a string literal.
But are "error" and 'error' equal in value and type? Let's find out;
<?php
$dblquotes = "error";
$sglquotes = 'error';
echo 'Is value equal?';
echo ($dblquotes == $sglquotes);
echo 'Are value and type equal?';
echo ($dblquotes === $sglquotes);
?>
They are equal in value and type - so this is not the issue.
Next, we need to see whether 'error' is a defined constant in PHP (it may not be a reserved word, but it could be a constant).
<?php
echo defined('error');
?>
No - it is not a defined constant.
Next, we can check to see whether 'error' equates to zero;
<?php
echo ('error' == 0);
?>
... which it does ...
This isn't documented anywhere that I could find (ie in the comparison tables for loose and strict comparisons - http://au3.php.net/manual/en/types.comparisons.php)
So - I think the result is that there is an 'undocumented feature' here - unless anyone else knows otherwise?
Cheers,
K.
Oh, and to get the PHP code displaying properly with syntax highlighting, enclose the PHP code in left square bracket, the word php, right square bracket then to end the PHP code, left square bracket, forward slash, the word php then right square bracket - can't find out how to demonstrate it without it formatting the code
[Updated on: Tue, 17 July 2007 08:09]
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| Re: Comparison operators: When equals doesn't equal equals [message #1785 is a reply to message #1140 ] |
Sat, 19 January 2008 18:29 |
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I think there is a better way to consolidate them, but I don't know what it is! We've kicked the idea around on the IRC channel before as well, without reaching a conclusion.
Personally I think there is some really good info buried on the forums and perhaps we could expand the front-end of the site to make it all more accessible. Anyone got a response or more specific suggestion?
Lorna Mitchell (online at http://www.lornajane.net)
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