Cannot imbed field variable in string when using single apostrophe

For some strange reason this

$SQL = 'SELECT field FROM table WHERE ID = {field}'; end up to be

SELECT field FROM table WHERE ID = $this->field

But

$SQL = "SELECT field FROM table WHERE ID = {field}"; work fine

SELECT field FROM table WHERE ID = x

Usually you do not use single qoute with query line in PHP it should be double qoute. you may use single qoute with field names but not the whole select statment.

Yes, you are right

Thanks