First create a default definition file.

<?php
define
('SAMPLE_HELLO''Hey');
define('SAMPLE_EX''This is example number %d!');
?>
Then create your PHP script which utilizes those defines.

<?php
switch ($argv[1]) {
    case 
'de':
        include_once(
'./simple_defs_de.php');
        break;
    case 
'en':
    default:
        include_once(
'./simple_defs_en.php');
        break;
}

print 
SAMPLE_HELLO;
print 
"\n<br>\n";
print 
sprintf(SAMPLE_EX1);
?>
Then copy over your english translation to the language translation you wish to make (in this case German).

]$ cp sample_defs_en.php sample_defs_de.php
And then edit the file, with the desired translations.

<?php
define
('SAMPLE_HELLO''Hoi');
define('SAMPLE_EX''Dass ist Beispiel Nummer %d!');
?>