I saw some people use this syntax in their code in the index.php or in the configuration file

```
defined('SOME_CONSTANT') || define('SOME_CONSTANT','SOME_VALUE');

```

while I understand what it means (if defined leaved it as it is, otherwise define it), but why to look if it is defined or not if it will be undefined at the beginning of the script ?

AFAIK PHP re-executes the script on every single request, so what's the point from using "defined".