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PHP 4 has no built-in json_encode() and cannot run PDO. For JSON, PEAR Services_JSON is a historically documented userland option; for database access, PEAR DB and MDB2 are historical alternatives worth checking. None should be assumed to work with a PHP 4 installation until its exact package release and required database driver have been verified. If you can upgrade the runtime, do that rather than building new dependencies around PHP 4.
What can you use instead of json_encode() in PHP 4?
json_encode() is documented for PHP 5 beginning with version 5.2.0, so it is not a native PHP 4 function. The current PHP manual describes recursive encoding of arrays and objects, requires string data to be UTF-8, and says the function returns a JSON string on success or false on failure. Its modern flags and depth parameters should not be assumed to apply to early PHP versions. PHP Manual: json_encode()
Historical option: PEAR Services_JSON
PEAR Services_JSON documents userland encode() and decode() methods, including examples with nested values. It says strings passed to encode() should be ASCII or UTF-8. That makes it a concrete historical alternative to investigate, not a blanket guarantee of PHP 4 compatibility: confirm the specific archived package release against the target runtime and test the input and output your application uses. PEAR Services_JSON API
Do not substitute PHP’s serialize() when a client or service expects JSON. PHP serialization is not a drop-in JSON wire format. Use a JSON implementation compatible with the other end of the exchange, or upgrade PHP so the native JSON extension is available.
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What replaces PDO for database access in PHP 4?
PDO is not a PHP 4 option. A legacy PDO introduction says it relies on PHP 5 object features and will not run on earlier versions. PDO also requires a driver for the particular database; it is an access interface, not a complete abstraction layer that rewrites SQL or supplies missing database features. Legacy PDO introduction · PHP Manual: PDO
PEAR DB and MDB2
PEAR DB and MDB2 are historically relevant database abstraction libraries. They offer common APIs across database backends, but each still depends on a backend driver, and support differs by driver and package version. The PEAR documentation explicitly marks some backends as PHP 5 only: MDB2 identifies MySQLi and Interbase/Firebird that way, while PEAR DB identifies MySQLi as requiring PHP 5. Do not infer that another listed backend is PHP 4-compatible without checking its exact release and driver. PEAR MDB2 feature overview · PEAR DB backend documentation
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ADOdb
ADOdb is another database abstraction library, but its current documentation describes modern PHP baselines and native database-driver requirements; it does not establish support for PHP 4. Consider it for a PHP 4 application only if an archived release’s requirements can be verified for the exact runtime and backend. ADOdb documentation
How to choose a legacy package safely
Before adopting any of these libraries in a PHP 4 application, verify the combination rather than relying on a package name alone:
- Runtime: Find the precise PHP version supported by the specific package release.
- Driver: Confirm the deployed PHP build has the database’s required native extension or driver.
- Query safety: Check the release- and driver-specific behavior for parameter binding and escaping; do not assume modern guarantees.
- Database needs: Check transaction support and whether the portability offered across backends matters to your application.
- Upgrade path: Consider whether adopting a legacy API will make a later runtime migration harder.
Package documentation establishes that drivers are needed and backend compatibility varies; it does not prove that every historical package-and-driver combination runs under every PHP 4 build. Test the exact deployment environment before relying on one.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why upgrading is the better long-term route
PHP 4-to-5 migration was not necessarily transparent: the migration material, for example, notes that MySQL support was no longer enabled by default in PHP 5. Inventory extensions, database access, and application behavior before upgrading rather than assuming the old setup carries over unchanged. PHP 4 to PHP 5 migration material
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On current PHP, the JSON extension cannot be disabled: the RFC targeting PHP 8.0 records that change as implemented. That removes the need for a JSON polyfill on those runtimes, but does nothing for an application that must remain on PHP 4. PHP RFC: Always available JSON extension
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