PHP quirks and pitfalls

Question

I've realized that, although most of my experience consists in writing PHP applications, I find myself making "beginner mistakes" from time to time. This because PHP is a language that has grown very organically and as such has some idiosyncrasies, quirks and pitfalls that I don't know about.

I'd like this question to be a wiki for all those who want to know PHP's pitfalls and exceptions from what we might believe are the rules. But please, don't write general responses like:

Some functions receive arguments as $needle, $haystack, while some as $haystack, $needle.

Tell the function names. You have a few answers from me as examples. Oh, and add one pitfall per answer. This way we'll see which is the most despised of them all (by voting).

I don't want to start a flame war, keep it to the subject. If you'd like to write some bad things about PHP then do it as a comment to the respective answer.

Hopefully this wiki will be of help for all of us, beginners and experts.

Update:

After Andrew Moore's comment I'm thinking that the answer should also contain a solution or workaround for the pitfall.

Answer 1

Serializing objects that process XML structures and then unserializing them does not restore the original XML structure:

$dom = new DOMDocument;
$dom->loadXML('<test/>');

$dom = serialize($dom);
$dom = unserialize($dom);

var_dump($dom->saveXML());
// $ Warning: DOMDocument::saveXML(): Couldn't fetch DOMDocument in ...
// $ NULL

Similarly for SimpleXML objects.

Answer 2

I hate how you can get issues if you close php files with a ?> closing tag (which seems like it should be the other way around). For example, include a file with some whitespace after the ?> and then try to change headers (assuming no output buffering). UGH. Took me way to long to learn to never close php files with ?>

Answer 3

Non-industry-standard operator precedence.

$x = 1;
echo 'foo: ' . $x+1 . ' bar';

Will output: "1 bar", instead of the expected "foo: 2 bar". Solution: Use parenthesis.

Answer 4

Integer size is platform dependent. You can't normally use 64bit integers on a 32bit machine without any outside module. Additionally, you cannot declare unsigned integers.

Answer 5

Multiple ways of doing truth tests (not operator, empty(), is_null(), isset()) + weak typing = this

With some discipline you can mostly avoid the need to refer to this table:

• For general truth tests, you can use boolean comparison if ($) { ... } if (!$x) { ... }. It behaves the way boolean operators in most languages do.

• Always use empty() if you want to test form input for falsy values (it treats "0" as false).

• Always use isset() if you want to determine whethere a variable is set or not

• Use is_null() or $x === NULL if you only need to check for NULL

Answer 6

I've had all sorts of bother combining foreach with references

$testarray = array(1 => "one", 2 => "two");
$item = "three";
$testarray[3] =& $item;
foreach ($testarray as $key => $item) {
  // do nothing
}
echo $testarray[3]; // outputs "two"

This really threw me off during the PHP4 era, and although it's gotten better in PHP5 by having sane behavior if you don't use explicit references, I still manage to get caught by this one now and then.

Answer 7

Inconsistent naming conventions of built-in functions. For example this set of string processing functions:

 str_shuffle()
 str_split()
 str_word_count()
 strcasecmp()
 strchr()
 strcmp()

Solution: Leave the manual open at all times.

Answer 8

Sometimes you like to pass a function-result to another function directly, which only accepts variables as input.

For example:

array_pop(  explode('\\', get_class($this))  )

…will result in an E_STRICT-error "Only variables should be passed by reference". To get get it working just add another pair of brackets around the inner (returned) statement:

array_pop( (explode('\\', get_class($this))) )

(If your are using namespaces and extended classes, this line returns only the class name (without namespace) from the calling object–not the parent-class which was extended and may hold this function.)

Answer 9

Some newly introduced PHP features fall flat because there's no guarantee they'll be supported by default in hosting environments.

The biggest peeve of mine is the short_tags setting that enables <? foobar(); ?> and <?=$var ?> tag syntax. I argue PHP should've enabled this feature by default, rather than opt-in.

== EDIT ==

In PHP >= 5.4 the short_tags setting is no longer taken into consideration for the shorthand echo statement , as such it will be available in every hosting environment that supports PHP 5.4 and up.

Answer 10

Having functions behave differently on different OS'es, and some functions are available only on specific OS'es.

For example, the mail() function on Windows cannot take sender name in the $to parameter. It can only contain email address. On Linux all works fine.

Another example, the function strptime() is only available on Linux, and not Windows (this got me in an existing project I wanted to run on my Windows box).

Sure, there are some functions that make sense only on certain OS'es (like Win32API functions), but many others seem like they should behave the same on all OS'es, when in fact they do not.

Answer 11

Although strings may be iterated using for loops and indexes, they can't be iterated using foreach loops. Example:

$str = 'foo';
$max = strlen($str);

for ($i=0; $i<$max; $i++)  {
    echo $str[$i];
}
// outputs: foo

foreach ($str as $char) {
    echo $char;
}
// Warning: Invalid argument supplied for foreach() ...

Answer 12

my favorite:

<?php
$a = 0;
$b = 'x';
var_dump(FALSE == $a);
var_dump($a == $b);
var_dump($b == TRUE);

echo' <br />Conclusion: TRUE equals FALSE (at least in PHP)';

Answer 13

== EDIT == This is no longer true for PHP >= 5.5!

What I sometimes run into is the Fatal error: Can't use function return value in write context error when using the empty() construct. For example:

if (!empty(trim($_GET['s']))) {
    // ...
}

empty() needs a variable, anything else will result in the error.

The solution:

$s = trim($_GET['s']);
if (!empty($s)) {
    // ...
}

Answer 14

$x = "a string";

if ($x == 142)
{
    echo "lol, of course $x is a number";
}
else
{
    echo "$x is clearly not equal to 142";
}

When run:

lol, of course a string is a number

You must use ===

$x = "a string";

if ($x === 142)
{
    echo "lol, of course $x is a number";
}
else
{
    echo "$x is clearly not equal to 142";
}

When run:

a string is clearly not equal to 142