DZone’s “PHP 5.5 & 5.6: What’s New in PHP” is Refcard #205, a free PDF by Luis Atencio covering language changes, platform enhancements and incompatibilities in PHP 5.5 and 5.6. It remains useful for understanding code written for those versions, but it is a historical reference—not a current PHP manual, security guide or deployment recommendation. PHP 5.5 and 5.6 are obsolete branches; check the PHP supported versions page before choosing a runtime.
What is the DZone PHP 5.5 and 5.6 Refcard?
The Refcard is a compact historical overview of changes introduced in PHP 5.5 and 5.6. DZone identifies it as Refcard #205 and credits Luis Atencio. Its table of contents covers new PHP 5.5 features, PHP 5.5 platform enhancements, and incompatibilities in both releases, followed by a conclusion. The landing page presents the card as a free downloadable PDF: DZone’s PHP 5.5 & 5.6 Refcard page.
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It is most useful when reading or maintaining legacy code, recalling transition-era syntax, or making an initial list of changes to investigate during an upgrade. It is not a complete migration plan, a current compatibility matrix, or guidance for selecting a supported production runtime.
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What PHP 5.5 introduced
Generators and incremental iteration
Generators let a function yield values as iteration proceeds instead of returning a completed array. For sequential processing, that can avoid holding the entire result set in memory. It does not make an inefficient data source efficient: if the function first loads every record into an array, the potential memory benefit is lost. Generators are chiefly a control-flow and memory-management feature, not a promise of faster execution.
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function numbers($max) {
for ($i = 1; $i <= $max; $i++) {
yield $i;
}
}
foreach (numbers(3) as $number) {
echo $number;
}See the PHP manual’s generator reference.
Cleanup with finally
A finally block runs after the try and any applicable catch handling, making it suitable for cleanup that must happen whether work succeeds or fails.
try {
// Work that may fail.
} catch (Exception $e) {
// Handle the exception.
} finally {
// Cleanup.
}Review control flow carefully when returns or exceptions are involved: an exception thrown in finally can affect the outcome of the surrounding operation. Consult the PHP exceptions documentation when adapting cleanup logic.
Password hashing APIs
PHP 5.5 added a dedicated API for password hashing and verification. Hashing is not reversible encryption: the application stores a one-way hash and verifies a submitted password against it.
$hash = password_hash($password, PASSWORD_DEFAULT);
if (password_verify($password, $hash)) {
// Password is valid.
}Store the complete hash returned by password_hash(); let the API manage salt generation rather than supplying a manually generated salt. After a successful login, password_needs_rehash() can identify hashes that should be upgraded to the current algorithm or settings. A stronger hash cannot be generated from the stored hash alone; the plaintext password is needed, which is why upgrades commonly happen after successful authentication. See the manual pages for password_hash(), password_verify(), and password_needs_rehash().
Array helpers and syntax improvements
array_column() extracts a field from an array of rows and can optionally use another field as the result keys:
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$users = [
['id' => 10, 'name' => 'A'],
['id' => 11, 'name' => 'B'],
];
$ids = array_column($users, 'id');Check the target runtime’s behavior for missing fields, object properties, duplicate values and the optional index argument. The manual documents array_column(). PHP 5.5 also added features such as ClassName::class, array and string dereferencing, list() in foreach, and improvements to empty(); the official PHP 5.5 new features guide is the better place to verify their exact behavior.
OPcache
PHP 5.5 bundled Zend OPcache, which caches compiled script bytecode so PHP does not need to parse and compile the same scripts on every request. The performance effect depends on workload, configuration, filesystem, deployment and PHP build. Treat any fixed speedup quoted in historical material as workload-specific, not a guarantee. Use the OPcache manual for configuration relevant to the PHP version actually deployed; do not copy old settings blindly.
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Variadic functions and argument unpacking
A variadic parameter, written with ..., collects remaining positional arguments into an array and must be the final parameter:
function sum($first, ...$numbers) {
return $first + array_sum($numbers);
}Argument unpacking uses an array to supply positional arguments to a function:
$values = [2, 3, 4];
function add($a, $b, $c) {
return $a + $b + $c;
}
echo add(...$values);These examples use PHP 5.6-era syntax. Newer PHP releases expanded argument-unpacking behavior, and later releases added type and return declarations not available in PHP 5.6. Check the target version rather than assuming a modern manual example runs unchanged. See PHP function arguments documentation.
Constant expressions and imports
PHP 5.6 allowed more scalar expressions in constant contexts, including arithmetic and concatenation involving constants and scalar values:
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const ONE = 1;
const TWO = ONE + ONE;
class Example {
const THREE = TWO + 1;
}It also added imports for functions and constants, alongside existing namespace imports:
use function VendorLibraryhelper;
use const VendorLibraryVERSION;The official references describe PHP 5.6 new features and namespace imports.
Exponentiation
The ** operator performs exponentiation, and **= assigns an exponentiated value. Exponentiation is right-associative, so 2 ** 3 ** 2 is evaluated as 2 ** (3 ** 2), or 512—not as (2 ** 3) ** 2.
echo 2 ** 3; // 8
echo 2 ** 3 ** 2; // 512See the PHP manual’s arithmetic operators reference.
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Timing-resistant comparison, debugging and inspection
hash_equals() is intended for comparing secret strings in a way designed to reduce timing leakage. Pass the known secret as the first argument:
if (hash_equals($knownToken, $userToken)) {
// Token matches.
}It does not make weak token generation safe or address token storage, transport, replay or expiration. It is not a replacement for every string comparison. See hash_equals().
PHP 5.6 also introduced phpdbg, an interactive debugger SAPI, and the __debugInfo() method for controlling information shown when an object is inspected. phpdbg may suit command-line debugging, but it does not replace every team’s IDE integration, remote-debugging, profiling or tracing workflow. The manual covers phpdbg and the PHP 5.6 feature set.
What to verify in PHP 5.5 and 5.6 incompatibilities
PHP 5.5
PHP 5.5 deprecated the old mysql_* extension and the /e modifier to preg_replace(). The latter evaluates replacement text as PHP code; a typical replacement is a callback instead:
$result = preg_replace_callback(
'/(w+)/',
function ($matches) {
return strtolower($matches[0]);
},
$input
);See preg_replace_callback() and the official PHP 5.5 incompatibility guide. Also review changes affecting case-insensitive matching, Windows support, extensions and configuration. Database access, regular expressions and string handling merit explicit regression tests.
PHP 5.6
The PHP 5.6 migration guide covers changes including deprecated calls to non-static methods in static contexts, handling of invalid JSON literal casing by json_decode(), class-property array declarations, encoding defaults and extension behavior. mcrypt availability and behavior also require version-specific checking; do not infer its status from a brief mention in a refcard. Consult the PHP 5.6 incompatibility guide for precise details and test the application’s actual extensions and build.
In both versions, a clean parse does not establish that the application is safe or supported: framework and Composer constraints, database drivers, operating-system packages and extensions can independently block an upgrade. Deprecation notices deserve investigation because code that still runs may later fail or disappear.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to use the Refcard safely today
- Use it as an index, not final authority. Check feature and compatibility claims against the official PHP 5.5 migration guide, PHP 5.6 migration guide, and the specific PHP manual page for the feature.
- Verify examples against the target interpreter. Keep PHP 5.5/5.6 syntax distinct from newer syntax such as scalar or return type declarations, null coalescing, arrow functions and later argument-unpacking capabilities.
- Do not treat historical configuration or performance claims as defaults. OPcache configuration and results depend on the runtime and workload.
- Do not use it for present-day security or deployment decisions. Confirm supported-runtime status independently and plan to move beyond PHP 5.6 rather than making it a long-term destination.
The DZone page’s indexed rendering contains apparent transcription or presentation problems, including malformed snippets and a section label that groups new PHP 5.6 features under “incompatibilities.” Treat the original PDF and page as historical material and resolve technical questions with the official manual rather than reproducing questionable examples.
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Inventory the running environment
Run these generic inspection commands in the relevant environment; they report the PHP CLI binary and configuration, which may differ from the web server’s PHP setup:
php -v
php -m
php --iniRecord loaded extensions and compare CLI, web, worker and scheduled-job environments. Review dependencies, framework requirements, database drivers and deployment packages before selecting a target version.
Find known hazards and surface warnings
- Search application code for
mysql_calls and plan a database API migration. - Find
preg_replace()patterns using/eand replace them with callbacks after checking behavior. - Review static calls to non-static methods,
mcrypt_*use, JSON assumptions, encoding behavior and version-specific extension code. - Enable suitable error and deprecation reporting in a non-production environment, then triage logs rather than dismissing warnings.
Test the target and prepare rollout
- Run the existing automated suite under the target runtime and resolve failures before deployment.
- Test database operations, sessions, authentication, file uploads, encoding, cron jobs, CLI commands and queue workers.
- Test extension-dependent paths independently, including drivers and libraries used by the application.
- Deploy through a staged rollout with monitoring and a rollback plan; verify logs and application behavior after each step.
Exact package-manager commands depend on operating system, repository, architecture and deployment method. If reproducing a PHP 5.x environment is necessary for investigation, isolate it in a container, virtual machine, dedicated legacy host or pinned CI job; isolation does not remove the maintenance and security risks of running an obsolete interpreter.
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