为什么strpos + strlen在PHP中不安全（例如：完全匹配）

I had a discussion with my teacher about the mb_ functions. Whatever, one thing leading to another, we changed the subject and he gave me an example where strpos and strlen could be problematic, according to him:

$input  = "something"; # input given by the user
$string = "hello";     # string to match

if ( strpos($input, $string) !== false && strlen($input) < strlen($string) ) {
    echo "Correct input";
} else {
    echo "Incorrect input";
}

(The question is not about how to match 2 strings)

According to my teacher, there may be a way to validate the statement and execute the code echo "Correct input";.

However, I can't see a flaw in this. Maybe there could be a problem with encoding? Do you have any idea?

okey i think that the flaw will be the if statement logic

how will you use strpos function which check the position of the first occurrence of a substring in a string , and in the same time you want to check if the input is greater than the subject ?

by logic it is impossible