Insert into a table which has a dash in the name

Enclosing identifiers with backtick makes reserved words/characters as valid identifiers in mysql.

So you should use

 `concept-relation`

The backtick (`)is the key at the top-left of this keyboard.

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$query2 = "INSERT INTO `concept-relation` (relationID, firstConceptID, secondConceptID)
                VALUES (:rID, :sID, :dID)";

Try:

"INSERT INTO `concept-relation` (relationID, firstConceptID, secondConceptID) VALUES (:rID, :sID, :dID)";

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