In PHP, both require() and require_once() are used to include external files into a PHP script. However, there is a subtle difference between the two.

The require() function is used to include a file and will throw a fatal error if the file cannot be found or included. If the same file is included multiple times using require(), PHP will throw a “Cannot redeclare” error.

On the other hand, the require_once() function is used to include a file only once. If the same file is included multiple times using require_once(), PHP will include it only once and ignore subsequent calls. This prevents any redeclaration errors.

In terms of performance, require_once() can be slower than require() when including multiple files. This is because require_once() needs to keep track of the files it has already included to avoid duplicates. However, the performance difference is usually negligible unless you are including a large number of files.

If you are including a small number of files or if you are unsure whether a file has already been included, it is generally safe to use require_once(). However, if you are including a large number of files and you are certain that there are no duplicates, using require() can provide a slight performance boost.

The main difference between require() and require_once() is that require_once() ensures that a file is included only once, while require() includes a file without any duplicate checks. The performance difference between the two is usually minimal, but it is worth considering when including a large number of files.