You should have read the fucking manual ..
There are 4 functions that do that in PHP:
include
require
include_once
require_once
And you can also use a 5th function called
readfile
wich reads a file and outputs it.
Like
<?= readfile ( "your.php" ); ?>
<? include ( "your.php" ); ?><? require ( "your.php" ); ?>
Etc..