PHP - how to create a newline character?

Use the predefined PHP_EOL constant:

echo $clientid, ' ', $lastname, PHP_EOL;

The constant value will be set according to the line endings of the operating system where PHP is executing. On Linux, it will be "\n"; on Windows, it will be "\r\n".

Only double quoted strings interpret the escape sequences \r and \n as '0x0D' and '0x0A' respectively, so you want:

"\r\n"

Single quoted strings, on the other hand, only know the escape sequences \\ and \'.

So unless you concatenate the single quoted string with a line break generated elsewhere (e. g., using double quoted string "\r\n" or using chr function chr(0x0D).chr(0x0A)), the only other way to have a line break within a single quoted string is to literally type it with your editor:

$s = 'some text before the line break
some text after';

Make sure to check your editor for its line break settings if you require some specific character sequence (\r\n for example).