Original value of $a is 21 and $b is 15.

The value of 21 == 15 is 0.
That is, 21 is not equal to 15.

The value of 21 != 15 is 1.
That is, 21 is not equal to 15.

The value of 21 > 15 is 1.
That is, 21 is greater than 15.

The value of 21 < 15 is 0.
That is, 21 is not less than 15.

The value of 21 >= 15 is 1.
That is, 21 is greater than or equal to 15.

The value of 21 <= 15 is 0.
That is, 21 is not less than or equal to 15.

Original value of $s is "Hello" and $t is 'hello'.

The value of "Hello" == 'hello' is 0.
That is, "Hello" is not equal to 'hello'.

The value of "Hello" != 'hello' is 1.
That is, "Hello" is not equal to 'hello'.

The value of "Hello" > 'hello' is 0.
That is, "Hello" is greater than 'hello'.

The value of "Hello" < 'hello' is 1.
That is, "Hello" is not less than 'hello'.

The value of "Hello" >= 'hello' is 0.
That is, "Hello" is greater than or equal to 'hello'.

The value of "Hello" <= 'hello' is 1.
That is, "Hello" is not less than or equal to 'hello'.