What potential issues arise when running a PHP3 project on a PHP4 server?

When running a PHP3 project on a PHP4 server, potential issues may arise due to deprecated functions and syntax differences between the two versions. To solve this, you can update the PHP3 code to be compatible with PHP4 by replacing deprecated functions and updating syntax to adhere to PHP4 standards.

// Example of updating PHP3 code to be compatible with PHP4

// PHP3 code using deprecated function
$oldVar = ereg_replace("old", "new", $string);

// Updated PHP4 code using preg_replace instead of ereg_replace
$newVar = preg_replace("/old/", "new", $string);