php_pitfalls.diviner

@title PHP Pitfalls
@group php

This document discusses difficult traps and pitfalls in PHP, and how to avoid,
work around, or at least understand them.

= array_merge() in Incredibly Slow When Merging A List of Arrays =

If you merge a list of arrays like this:

  COUNTEREXAMPLE
  $result = array();
  foreach ($list_of_lists as $one_list) {
    $result = array_merge($result, $one_list);
  }

...your program now has a huge runtime because it generates a large number of
intermediate arrays and copies every element it has previously seen each time
you iterate.

In a libphutil environment, you can use @{function@libphutil:array_mergev}
instead.

= var_export() Hates Baby Animals =

If you try to var_export() an object that contains recursive references, your
program will terminate. You have no chance to intercept or react to this or
otherwise stop it from happening. Avoid var_export() unless you are certain
you have only simple data. You can use print_r() or var_dump() to display
complex variables safely.

= isset(), empty() and Truthiness =

A value is "truthy" if it evaluates to true in an ##if## clause:

  $value = something();
  if ($value) {
    // Value is truthy.
  }

If a value is not truthy, it is "falsey". These values are falsey in PHP:

  null      // null
  0         // integer
  0.0       // float
  "0"       // string
  ""        // empty string
  false     // boolean
  array()   // empty array

Disregarding some bizarre edge cases, all other values are truthy. Note that
because "0" is falsey, this sort of thing (intended to prevent users from making
empty comments) is wrong in PHP:

  COUNTEREXAMPLE
  if ($comment_text) {
    make_comment($comment_text);
  }

This is wrong because it prevents users from making the comment "0". //THIS
COMMENT IS TOTALLY AWESOME AND I MAKE IT ALL THE TIME SO YOU HAD BETTER NOT
BREAK IT!!!// A better test is probably strlen().

In addition to truth tests with ##if##, PHP has two special truthiness operators
which look like functions but aren't: empty() and isset(). These operators help
deal with undeclared variables.

In PHP, there are two major cases where you get undeclared variables -- either
you directly use a variable without declaring it:

  COUNTEREXAMPLE
  function f() {
    if ($not_declared) {
      // ...
    }
  }

...or you index into an array with an index which may not exist:

  COUNTEREXAMPLE
  function f(array $mystery) {
    if ($mystery['stuff']) {
      // ...
    }
  }

When you do either of these, PHP issues a warning. Avoid these warnings by using
empty() and isset() to do tests that are safe to apply to undeclared variables.

empty() evaluates truthiness exactly opposite of if(). isset() returns true for
everything except null. This is the truth table:

  VALUE             if()        empty()     isset()

  null              false       true        false
  0                 false       true        true
  0.0               false       true        true
  "0"               false       true        true
  ""                false       true        true
  false             false       true        true
  array()           false       true        true
  EVERYTHING ELSE   true        false       true

The value of these operators is that they accept undeclared variables and do not
issue a warning. Specifically, if you try to do this you get a warning:

  COUNTEREXAMPLE
  if ($not_previously_declared) {         // PHP Notice:  Undefined variable!
    // ...
  }

But these are fine:

  if (empty($not_previously_declared)) {  // No notice, returns true.
    // ...
  }
  if (isset($not_previously_declared)) {  // No notice, returns false.
    // ...
  }

So, isset() really means is_declared_and_is_set_to_something_other_than_null().
empty() really means is_falsey_or_is_not_declared(). Thus:

  - If a variable is known to exist, test falsiness with if (!$v), not empty().
    In particular, test for empty arrays with if (!$array). There is no reason
    to ever use empty() on a declared variable.
  - When you use isset() on an array key, like isset($array['key']), it will
    evaluate to "false" if the key exists but has the value null! Test for index
    existence with array_key_exists().

Put another way, use isset() if you want to type "if ($value !== null)" but are
testing something that may not be declared. Use empty() if you want to type
"if (!$value)" but you are testing something that may not be declared.

= usort(), uksort(), and uasort() are Slow =

This family of functions is often extremely slow for large datasets. You should
avoid them if at all possible. Instead, build an array which contains surrogate
keys that are naturally sortable with a function that uses native comparison
(e.g., sort(), asort(), ksort(), or natcasesort()). Sort this array instead, and
use it to reorder the original array.

In a libphutil environment, you can often do this easily with
@{function@libphutil:isort} or @{function@libphutil:msort}.

= array_intersect() and array_diff() are Also Slow =

These functions are much slower for even moderately large inputs than
array_intersect_key() and array_diff_key(), because they can not make the
assumption that their inputs are unique scalars as the ##key## varieties can.
Strongly prefer the ##key## varieties.

= array_uintersect() and array_udiff() are Definitely Slow Too =

These functions have the problems of both the ##usort()## family and the
##array_diff()## family. Avoid them.

= foreach() Does Not Create Scope =

Variables survive outside of the scope of foreach(). More problematically,
references survive outside of the scope of foreach(). This code mutates
##$array## because the reference leaks from the first loop to the second:

  COUNTEREXAMPLE
  $array = range(1, 3);
  echo implode(',', $array); // Outputs '1,2,3'
  foreach ($array as &$value) {}
  echo implode(',', $array); // Outputs '1,2,3'
  foreach ($array as $value) {}
  echo implode(',', $array); // Outputs '1,2,2'

The easiest way to avoid this is to avoid using foreach-by-reference. If you do
use it, unset the reference after the loop:

  foreach ($array as &$value) {
    // ...
  }
  unset($value);

= unserialize() is Incredibly Slow on Large Datasets =

The performance of unserialize() is nonlinear in the number of zvals you
unserialize, roughly O(N^2).

  zvals       approximate time
  10000       5ms
  100000      85ms
  1000000     8,000ms
  10000000    72 billion years


= call_user_func() Breaks References =

If you use call_use_func() to invoke a function which takes parameters by
reference, the variables you pass in will have their references broken and will
emerge unmodified. That is, if you have a function that takes references:

  function add_one(&$v) {
    $v++;
  }

...and you call it with call_user_func():

  COUNTEREXAMPLE
  $x = 41;
  call_user_func('add_one', $x);

...##$x## will not be modified. The solution is to use call_user_func_array()
and wrap the reference in an array:

  $x = 41;
  call_user_func_array(
    'add_one',
    array(&$x)); // Note '&$x'!

This will work as expected.

= You Can't Throw From __toString() =

If you throw from __toString(), your program will terminate uselessly and you
won't get the exception.

= An Object Can Have Any Scalar as a Property =

Object properties are not limited to legal variable names:

  $property = '!@#$%^&*()';
  $obj->$property = 'zebra';
  echo $obj->$property;       // Outputs 'zebra'.

So, don't make assumptions about property names.

= There is an (object) Cast =

You can cast a dictionary into an object.

  $obj = (object)array('flavor' => 'coconut');
  echo $obj->flavor;      // Outputs 'coconut'.
  echo get_class($obj);   // Outputs 'stdClass'.

This is occasionally useful, mostly to force an object to become a Javascript
dictionary (vs a list) when passed to json_encode().

= Invoking "new" With an Argument Vector is Really Hard =

If you have some ##$class_name## and some ##$argv## of constructor
arguments and you want to do this:

  new $class_name($argv[0], $argv[1], ...);

...you'll probably invent a very interesting, very novel solution that is very
wrong. In a libphutil environment, solve this problem with
@{function@libphutil:newv}. Elsewhere, copy newv()'s implementation.

= Equality is not Transitive =

This isn't terribly surprising since equality isn't transitive in a lot of
languages, but the == operator is not transitive:

  $a = ''; $b = 0; $c = '0a';
  $a == $b; // true
  $b == $c; // true
  $c == $a; // false!

When either operand is an integer, the other operand is cast to an integer
before comparison. Avoid this and similar pitfalls by using the === operator,
which is transitive.

= All 676 Letters in the Alphabet =

This doesn't do what you'd expect it to do in C:

  for ($c = 'a'; $c <= 'z'; $c++) {
    // ...
  }

This is because the successor to 'z' is 'aa', which is "less than" 'z'. The
loop will run for ~700 iterations until it reaches 'zz' and terminates. That is,
##$c## will take on these values:

  a
  b
  ...
  y
  z
  aa // loop continues because 'aa' <= 'z'
  ab
  ...
  mf
  mg
  ...
  zw
  zx
  zy
  zz // loop now terminates because 'zz' > 'z'

Instead, use this loop:

  foreach (range('a', 'z') as $c) {
    // ...
  }