fn register_constant(&self, name: &str, module_number: i32) -> Result<()> {
        self.register_constant_flags(name, module_number, GlobalConstantFlags::Persistent)
    }
Registers a global module constant in PHP, with the value as the content
of self. This function _must_ be called in the module startup
function, which is called after the module is initialized. The
second parameter of the startup function will be the module number.
By default, the case-insensitive and persistent flags are set when
registering the constant.

Returns a result containing nothing if the constant was successfully
registered.

# Parameters

* `name` - The name of the constant.
* `module_number` - The module number that we are registering the
constant under.

# Errors

Returns an error if the constant could not be registered.

# Examples

```no_run
use ext_php_rs::constant::IntoConst;

pub extern "C" fn startup_function(_type: i32, module_number: i32) -> i32 {
5.register_constant("MY_CONST_NAME", module_number); // MY_CONST_NAME == 5
"Hello, world!".register_constant("STRING_CONSTANT", module_number); // STRING_CONSTANT == "Hello, world!"
0
}
```
src/constant.rs