Check If Preg Match False instead of True

You can negate the condition like this:

if (!preg_match('/^[A-Za-z0-9]+$/', $username))
{
    echo 'Not secure enough';
}

Also, your regex needs to be [A-Za-z0-9]+ if you mean "alphanumeric" (only letters and numbers) as a whole.

The regex in your code would match if the username 1) starts with a capital letter (or more than one) 2) is followed by one or more lower-case letter and 3) ends with one or more number(s).

Edit:

I'm really not sure if this is what you want. You can do, basically:

if (preg_match('/^[A-Za-z0-9]+$/', $username)) {
    echo 'Is only letters and numbers';
} else {
    echo 'Contains some other characters';
}

Do you want to make sure the string contains special characters so that it will be "secure enough"? Or do you want to make sure that it does not contains special characters, so there won't be any problems processing special characters at some point?

A secure password would be one with special characters and while you probably don't want to enforce this (depending on your target audience), you'd usually want your system to support special characters in passwords.

That's what the ! operator is for, so just say

if (!preg_match.....)

or of course this is what the else clause is for, either way this is rudimentary programming and you need to read a basic tutorial before asking such simple things.