You CANNOT bind parameters for table and/or column names. Binding parameters only works for values:
YES: $sqlQuery = "UPDATE ".TB_NAME." SET ".CN_NAME." = ? WHERE DAY = ?";
NO: $sqlQuery = "UPDATE ? SET ? = ? WHERE DAY = ?";
Any attempt to bind column or table names will result in the following error during db2_execute():
PHP Warning: db2_bind_param(): Describe Param Failed in ...