PL/SQL, how to escape single quote in a string?

You can use literal quoting:

stmt := q'[insert into MY_TBL (Col) values('ER0002')]';

Documentation for literals can be found here.

Alternatively, you can use two quotes to denote a single quote:

stmt := 'insert into MY_TBL (Col) values(''ER0002'')';

The literal quoting mechanism with the Q syntax is more flexible and readable, IMO.

Here's a blog post that should help with escaping ticks in strings.

Here's the simplest method from said post:

The most simple and most used way is to use a single quotation mark with two single quotation marks in both sides.

SELECT 'test single quote''' from dual;

The output of the above statement would be:
test single quote'

Simply stating you require an additional single quote character to print a single quote character. That is if you put two single quote characters Oracle will print one. The first one acts like an escape character.

This is the simplest way to print single quotation marks in Oracle. But it will get complex when you have to print a set of quotation marks instead of just one. In this situation the following method works fine. But it requires some more typing labour.

In addition to DCookie's answer above, you can also use chr(39) for a single quote.

I find this particularly useful when I have to create a number of insert/update statements based on a large amount of existing data.

Here's a very quick example:

Lets say we have a very simple table, Customers, that has 2 columns, FirstName and LastName. We need to move the data into Customers2, so we need to generate a bunch of INSERT statements.

Select 'INSERT INTO Customers2 (FirstName, LastName) ' ||
       'VALUES (' || chr(39) || FirstName || chr(39) ',' || 
       chr(39) || LastName || chr(39) || ');' From Customers;

I've found this to be very useful when moving data from one environment to another, or when rebuilding an environment quickly.