You could also say:
SELECT wrd FROM tablename WHERE wrd LIKE CONCAT(:partial, '%')
to do the string joining at the MySQL end, not that there’s any particular reason to in this case.
Things get a bit more tricky if the partial wrd
you are looking for can itself contain a percent or underscore character (since those have special meaning for the LIKE operator) or a backslash (which MySQL uses as another layer of escaping in the LIKE operator — incorrectly, according to the ANSI SQL standard).
Hopefully that doesn’t affect you, but if you do need to get that case right, here’s the messy solution:
$stmt= $db->prepare("SELECT wrd FROM tablename WHERE wrd LIKE :term ESCAPE '+'");
$escaped= str_replace(array('+', '%', '_'), array('++', '+%', '+_'), $var);
$stmt->bindParam(':term', $escaped);