Why i should use dot in the above line before and after $num

Question

echo "Counter is ".$num."<br />";

Answer 1

to separate variable from string. Your string is constant, but variables can be dynamic. For more information comment

Answer 2

with the dot you concat strings so in this line you put the two strings and the var $num into one big string and then bring it to the output.
you could also use: echo "Counter is $num <br/>";
this would inject the value of $num in the String.
but you have to use the doubled " !
this does not work with single '