What are the key differences between PHP 4 and PHP 5 libraries and how can users ensure compatibility when working with different versions?

The key differences between PHP 4 and PHP 5 libraries include changes in function names, parameter requirements, and new features introduced in PHP 5. To ensure compatibility when working with different versions, users can utilize conditional statements to check the PHP version and adjust their code accordingly. 

if (version_compare(PHP_VERSION, '5.0.0', '<')) {
    // PHP 4 code here
} else {
    // PHP 5 code here
}

Keywords

PHP 4 PHP 5 libraries compatibility version control

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