Here-docs are just a convenient way of writing long strings, so (for example)

my_function(<<END, <<END . <<END);
a
END
b
END
c
END
[download]

is equivalent to

my_function("a\n", "b\n" . "c\n");
[download]

You can of course use different names, i.e.

my_function(<<A, <<B . <<C);
a
A
b
B
c
C
[download]

(well, that's not really sensible...)

Note that the here-docs must be in order, and start at the end of the line, i.e.

printf <<'END', 1, 2,
%d %d %d %d
END
3, 4;
[download]

prints 1 2 3 4.