Aus dem PDO Manual zu bindParam:

„Binds a PHP variable to a corresponding named or question mark placeholder in the SQL statement that was used to prepare the statement. Unlike PDOStatement::bindValue(), the variable is bound as a reference and will only be evaluated at the time that PDOStatement::execute() is called.

Most parameters are input parameters, that is, parameters that are used in a read-only fashion to build up the query. Some drivers support the invocation of stored procedures that return data as output parameters, and some also as input/output parameters that both send in data and are updated to receive it.“

Scheint also für Rückgabewerte von Stored Procedures gedacht, wenn ich das richtig verstehe.

Gruß
Dennis