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_EX, 1);
?>
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!');
?>