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# Chapter 55. Migrating from PHP 3 to PHP 4

PHP has already a long history behind him : Legendary PHP 1.0, PHP/FI, PHP 3.0 and PHP 4.0.

1. Migrating from PHP3 to PHP4
2. Incompatible functions?

1. Migrating from PHP3 to PHP4

PHP 4 was designed to be as compatible with earlier versions of PHP as possible and very little functionality was broken in the process. If you're really unsure about compatibility you should install PHP 4 in a test environment and run your scripts there.

Also see the appropriate migration appendix of this manual.

2. Incompatible functions?

Since PHP 4 is basically a rewrite of the entire PHP engine there was very few functions that were altered and only then some of the more exotic ones.

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Migrating from PHP 2 to PHP 3 | Up | Miscellaneous Questions