The basic rules I use with ', ` and " are:-
I only use ` in MySQL and only where the field is a reserved word i.e. you have a field called date so the query ends up like
SELECT title, `date` FROM sometable
I use ' and " within PHP for different things:
if we have a variable called $test with a value of "this is my sample data";
' spits out exactly what's within the apostrophes (variable aren't evaluated) so
echo 'My text is: $test'; will output
My text is: $test
" allows the variables to be evaluated so
echo "My text is: $test"; will output
My text is: this is my sample data
I hope this helps