set serveroutput on
declare
  a int;
begin
  execute immediate 'select employee_id from employees where first_name=:ab' into a using 'Donald' ;
  dbms_output.put_line(a);
end;

set serveroutput on
declare
  a int;
begin
  execute immediate 'select employee_id into :1 from employees where first_name=:2' using a,'Donald';
  dbms_output.put_line(a);
end;

declare
  a int;
begin
  select employee_id into a from employees where first_name='Donald';
  dbms_output.put_line(a);
end;

declare
  a int;
begin
  execute immediate 'begin'
                  ||' select employee_id into :1'
                  ||' from employees'
                  ||' where first_name=:2;'
                  ||' end;'
    using out a,'Donald';
  dbms_output.put_line(a);
end;