I know it's 50-50 at least one of these items will change before my mail
reaches the list, but here's version 4 for your perusal.
Note: I have type hints for arrays down as being 'still under discussion' -
this isn't actually ready to go.
I've thrown out new features such as the Zend VM execution modes (and am
still debating whether to throw out default class type hints) because they
should be covered elsewhere, and I haven't done anything about replaced
extensions (e.g. w32api => ffi) because they should be covered in the
manual. I've some fear of terrifying would-be upgraders with an
unnecessarily long list here :)
Please be critical.
Thanks,
- Steph
UPGRADE NOTES - PHP 5.1
1. Changes in reference handling
a. Overview
b. Code that worked under PHP 4.3, but now fails
c. Code that was valid under PHP 4.3, but now throws an error
d. Code that failed under PHP 4.3, but now works
e. Code that 'should have worked' under PHP 5.0
f. Warnings that came and went
2. Reading []
3. String offset access
4. instanceof, is_a(), is_subclass_of(), catch
5. Abstract private methods
6. Type hints and NULL* STILL UNDER DISCUSSION (for arrays)
7. Integer values in function parameters
8. General extension-related changes in PHP 5.1
9. Date/time support
10. Changes in database support
a. PDO overview
b. Changes in MySQL support
c. Changes in SQLite support
11. Further migration information
===============================================================================
1. Changes in reference handling
================================
1a. Overview
============
>From the PHP script writer's point of view, the change most likely to impact
legacy code is in the way that references are handled in all PHP versions
post-dating the PHP 4.4.0 release.
Until and including PHP 4.3, it was possible to send, assign or return variables
by reference that should really be returned by value, such as a constant, a
temporary value (e.g. the result of an expression), or the result of a function
that had itself been returned by value, as here:
<?php
$foo = "123";
function return_value() {
global $foo;
return $foo;
}
$bar = &return_value();
?>
Although this code would usually work as expected under PHP 4.3, in the general
case the result is undefined. The Zend Engine could not act correctly on these
values as references. This bug could and did lead to various hard-to-reproduce
memory corruption problems, particularly where the code base was large.
In PHP 4.4.0, PHP 5.0.4 and all subsequent PHP releases, the Engine was fixed
to 'know' when the reference operation is being used on a value that should
not be referenced. The actual value is now used in such cases, and a warning
is emitted. The warning takes the form of an E_NOTICE in PHP 4.4.0 and up,
and E_STRICT in PHP 5.0.4 and up.
Code that could potentially produce memory corruption can no longer do so.
However, some legacy code might work differently as a result.
1b. Code that worked under PHP 4.3, but now fails
=================================================
<?php
function func(&$arraykey) {
return $arraykey; // function returns by value!
}
$array = array('a', 'b', 'c');
foreach (array_keys($array) as $key) {
$y = &func($array[$key]);
$z[] =& $y;
}
var_dump($z);
?>
Running the above script under any version of PHP that pre-dates the reference
fix would produce this output:
array(3) {
[0]=>
&string(1) "a"
[1]=>
&string(1) "b"
[2]=>
&string(1) "c"
}
Following the reference fix, the same code would result in:
array(3) {
[0]=>
&string(1) "c"
[1]=>
&string(1) "c"
[2]=>
&string(1) "c"
}
This is because, following the changes, func() assigns by value. The value
of $y is re-assigned, and reference-binding is preserved from $z. Prior
to the fix, the value was assigned by reference, leading $y to be
re-bound on each assignment. The attempt to bind to a temporary value
by reference was the cause of the memory corruption.
Such code can be made to work identically in both the pre-fix and the
post-fix PHP versions. The signature of func() can be altered to return
by reference, or the reference assignment can be removed from the result
of func().
<?php
function func() {
return 'function return';
}
$x = 'original value';
$y =& $x;
$y = &func();
echo $x;
?>
In PHP 4.3 $x would be 'original value', whereas after the changes it would
be 'function return' - remember that where the function does not return by
reference, the reference assignment is converted to a regular assignment.
Again, this can be brought to a common base, either by forcing func() to
return by reference or by eliminating the by-reference assignment.
1c. Code that was valid under PHP 4.3, but now throws an error
==============================================================
<?php
class Foo {
function getThis() {
return $this;
}
function destroyThis() {
$baz =& $this->getThis();
}
}
$bar = new Foo();
$bar->destroyThis();
var_dump($bar);
?>
In PHP 5.0.3, $bar evaluated to NULL instead of returning an object.
That happened because getThis() returns by value, but the value here
is assigned by reference. Although it now works in the expected way,
this is actually invalid code which will throw an E_NOTICE under
PHP 4.4 or an E_STRICT under PHP 5.0.4 and up.
1d. Code that failed under PHP 4.3, but now works
=================================================
<?php
function &f() {
$x = "foo";
var_dump($x);
print "$x\n";
return($a);
}
for ($i = 0; $i < 3; $i++) {
$h = &f();
}
?>
In PHP 4.3 the third call to var_dump produces NULL, due to the memory
corruption caused by returning an uninitialized value by reference.
This is valid code in PHP 5.0.4 and up, but threw errors in earlier
releases of PHP.
<?php
$arr = array('a1' => array('alfa' => 'ok'));
$arr =& $arr['a1'];
echo '-'.$arr['alfa']."-\n";
?>
Until PHP 5.0.5, it wasn't possible to assign an array element by
reference in this way. It now is.
1e. Code that 'should have worked' under PHP 5.0
================================================
There are a couple of instances of bugs reported under PHP 5.0 prior
to the reference fixes which now 'work'. However, in both cases errors
are thrown by PHP 5.1, because the code was invalid in the first place.
Returning values by reference using self:: now works in the general
case but throws an E_STRICT warning, and although your mileage may
vary when assigning by reference to an overloaded object, you will
still see an E_ERROR when you try it, even where the assignment
itself appears to work.
1f. Warnings that came and went
===============================
<?php
function & foo() {
$var = 'ok';
return $var;
}
function & bar() {
return foo();
}
$a =& bar();
echo "$a\n";
?>
Nested calls to functions returning by reference are valid code under both
PHP 4.3 and PHP 5.1, but threw an unwarranted E_NOTICE or E_STRICT under
the intervening PHP releases.
2. Reading []
=============
<?php
class XmlTest {
function test_ref(&$test) {
$test = "ok";
}
function test($test) { }
function run() {
$ar = array();
$this->test_ref($ar[]);
var_dump($ar);
$this->test($ar[]);
}
}
$o = new XmlTest();
$o->run();
?>
This should always have thrown a fatal E_ERROR, because [] cannot be
used for reading. It is invalid code in PHP 4.4.2 and PHP 5.0.5 upward.
3. String offset access
=======================
In PHP, both [] and {} can be used for accessing string offsets, e.g.
php -r "$str = "string"; echo $str{5}";
and
php -r "$str = "string"; echo $str[5]";
would both return the same result. This has led to many complaints over
inconsistent code in the past, and the [] syntax was deprecated some years
ago in an attempt to resolve the issue. However, it appears that [] is the
more popular means of accessing string offsets, so the decision has now
been made to deprecate the {} string offset syntax instead, with the
intention of removing it fully at a later date.
php -r "$str = "string"; echo $str{5}";
will now return an E_STRICT message to that effect in PHP 5.1.0 and up,
and you are strongly discouraged from using this syntax in new code.
4. instanceof, is_a(), is_subclass_of(), catch
==============================================
In PHP 5.0, is_a() was deprecated and replaced by the "instanceof" operator.
There were some issues with the initial implementation of "instanceof", which
relied on __autoload() to search for missing classes. If the class was not
present, "instanceof" would throw a fatal E_ERROR due to the failure of
__autoload() to discover that class. The same behaviour occurred in the
"catch" operator and the is_subclass_of() function, for the same reason.
None of these functions or operators call __autoload() in PHP 5.1, and
the workarounds used in PHP 5.0 are no longer necessary.
5. Abstract private methods
===========================
Abstract private methods were supported up until PHP 5.0.4, but were then
disallowed on the grounds that the behaviours of 'private' and 'abstract'
are mutually exclusive.
6. Default class type hints
===========================
>From PHP 5.1.0, it is possible to have a default value of NULL for parameters
with a class type hint.
The jury's still out over whether the same should be true of arrays.
7. Integer values in function parameters
========================================
With the advent of PHP 5.0, a new parameter parsing API was introduced
which is used by a large number of PHP functions. In all versions of
PHP between 5.0 and 5.1, the handling of integer values was very strict
and would reject non-well formed numeric values when a PHP function
expected an integer. These checks have now been relaxed to support
non-well formed numeric strings such as " 123" and "123 ". However, to
promote code safety and input validation, PHP functions will now emit
an E_NOTICE when such strings are passed as integers.
8. General extension-related changes in PHP 5.1
===============================================
One of the first things you're likely to notice when you download PHP 5.1 is
that
several of the older extensions have disappeared. The majority of the missing
extensions are still available in PECL, the PHP Extension Community Library, at
http://pecl.php.net. Windows binaries are built regularly, and you can obtain
the binaries for PECL extensions built against PHP 5.1 from
http://pecl4win.php.net/list.php/5_1.
The Zend Engine 2.1 API allows extension developers to declare class constants
in object oriented extension. New extensions written for PHP 5.1, including SPL,
PDO, ext/XMLReader and ext/date, have their constants in the format
PDO::CLASS_CONSTANT
rather than in the C format
PDO_CLASS_CONSTANT
in order to minimise pollution of the global namespace in PHP.
Note that the new Date class exists at this point purely to allow the core date
extension to adhere to the above convention, although extended functionality
is planned for the Date class in future.
9. Date/time support
====================
Date/time support has been fully rewritten in PHP 5.1, and no longer
uses the system settings to 'know' the timezone in operation. It will
instead utilize, in the following order:
* The timezone set using the date_default_timezone_set() function (if any)
* The TZ environment variable (if non empty)
* The date.timezone ini option (if set)
* "magical" guess (if the operating system supports it)
* If none of the above options succeeds, UTC
To ensure accuracy (and avoid an E_STRICT warning), you will need to define
your timezone in your php.ini using the following format:
date.timezone = Europe/London
The supported timezones are listed, in this format, in the PHP manual at
http://www.php.net/manual/en/timezones.php.
10. Changes in database support
==============================
10a. PDO overview
================
PHP Data Objects (PDO) were introduced as a PECL extension under PHP 5.0,
and became part of the core PHP distribution in PHP 5.1. The PDO extension
provides a consistent interface for database access, and is used alongside
database-specific PDO drivers. Each driver may also have database-specific
functions of its own, but basic data access functionality such as issuing
queries and fetching data is covered by PDO functions, using the driver
named in PDO::__construct().
You are encouraged to use PDO when creating new projects in PHP 5.1. Legacy
code will generally rely on the pre-existing database extensions, which are
still maintained.
Note that the PDO extension, and its drivers, are intended to be built as
shared extensions. This will enable straightforward driver upgrades from
PECL, without the need to rebuild PHP 5.1.
There is more in-depth information about the PDO extension in the manual
at http://www.php.net/manual/ref.pdo.php.
10b. Changes in MySQL support
============================
In PHP 4, MySQL 3 support was built-in. With the release of PHP 5.0 there
were two MySQL extensions, named 'mysql' and 'mysqli', which were designed
to support MySQL < 4.1 and MySQL 4.1 and up, respectively. With the
introduction of PDO, which provides a very fast interface to all the
database APIs supported by PHP, the PDO_MYSQL driver can support any
of the current versions (MySQL 3, 4 or 5) in PHP code written for PDO,
depending on the MySQL library version used during compilation. The
older MySQL extensions remain in place for reasons of back compatibility,
but are not enabled by default.
10c. Changes in SQLite support
=============================
In PHP 5.0, SQLite 2 support was provided by the built-in sqlite
extension, which was also available as a PECL extension in PHP 4.3
and PHP 4.4. With the introduction of PDO, the sqlite extension doubles
up to act as a 'sqlite2' driver for PDO; it is due to this that the
sqlite extension in PHP 5.1 has a dependency upon the PDO extension.
PHP 5.1 ships with a number of alternative interfaces to sqlite:
The sqlite extension provides the "classic" sqlite procedural/OO API
that you may have used in prior versions of PHP. It also provides the
PDO 'sqlite2' driver, which allows you to access legacy SQLite 2
databases using the PDO API.
PDO_SQLITE provides the 'sqlite' version 3 driver. SQLite version 3
is vastly superior to SQLite version 2, but the file formats of the
two versions are not compatible.
If your SQLite-based project is already written and working against
earlier PHP versions, then you can continue to use ext/sqlite without
problems, but will need to explicitly enable both PDO and sqlite. New
projects should use PDO and the 'sqlite' (version 3) driver, as this is
faster than SQLite 2, has improved locking concurrency, and supports
both prepared statements and binary columns natively.
11. Further migration information
================================
For general information about migrating from PHP 4 to PHP 5, please refer to
the relevant section in the PHP manual at
http://www.php.net/manual/migration5.php.
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