Both == and === are equality operator and check if the left and right values are equal. But, the === operator actually checks to see if the left and right values are equal, and also checks to see if they are of the same variable type (like whether they are both booleans, ints, etc, remember in PHP variable type is defined at the time of the usage).
Practical use
if (is_str_exist($a, $b) == false) // bad code
{
// do something
}
Now suppose is_str_exist returns falls if b or a is null, returns position of the string $a within $b which can be zero also but as it is == operator so the false == false is true and 0 == false is also true. Correct code should be
if (is_str_exist($a, $b) === false) // bad code
{
// do something
}
