How can PHP4 be made completely backwards compatible with PHP3?

To make PHP4 completely backwards compatible with PHP3, you would need to ensure that all deprecated functions and features from PHP3 are still supported in PHP4. This may involve using conditional statements to check for the version of PHP being used and implementing alternative functions or methods where necessary.

if (version_compare(PHP_VERSION, '4.0.0', '<')) {
    // Code to handle deprecated functions or features from PHP3
}