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Put them in backtick characters (see below the post field here) or in <code> tags

That’s a single quote or apostrophe, not a back tick. On a US English keyboard, the single quote is on the same key with a double quote. The back tick is in the upper left corner or the keyboard, usually on the same key as the tilde (tilde requires shift.)

http://upload.wikimedia.org/wikipedia/en/5/51/KB_United_States-NoAltGr.svg

You want to use the upper left key in this keyboard layout around your code.

<?php e_('This is code'); ?> and ‘this is just inside single quotes’.