为什么调用define（）中的函数会破坏我的网站？

Question

The following breaks my site quite badly:

define('FOO', get_foo());

However, the following works fine:

define('FOO', 'BAR');

Answer 1

Because define() expects a value for the constant and not the returned value of a function:

The value of the constant; only scalar and null values are allowed. Scalar values are integer, float, string or boolean values. It is possible to define resource constants, however it is not recommended and may cause unpredictable behavior.