In PHP can I check a boolean on a function call?

I have a function which checks the passed value and returns false or true, if false I want it to add something to an array. The code I've written is below.

if(!check_input($_POST['username'])){
    $errors[] = "Username";
}  

Right now it adds to my array anyway, regardless 开发者_开发百科of what is entered in the form. Is the way I've written that the correct way to check if the return from check_input() is false? I've checked the function's logic by altering the returns to echoes and it's returning the correct value, I'm just not sure if I'm checking it wrong.

I'd previously attempted to write it as $X=check_input etc, and then if(check_value == false) but that doesn't seem to give me the desired result either. Hmmm a quick pointer please!

Edit:

Okay, after looking at my function I think I spotted the problem, I have an if statement, and within that an if else statement, the first if checks the value isn't empty, the second if checks it doesn't exceed its maximum length and returns true/false. I set it to only check the max length if I'd passed that value in - or it's set to nothing. But, on balance I don't have a return true for the main if statement. So my logic is currently, if the value's bigger than nothing then check it against max length, then return true. If I don't check max length and it it does have value it's not returning anything. Hmmm. I'm thinking that's the cause of my issue?

Edit 2:

Thanks all, and to Marvin, who suggested I recheck the function. I've got it working. It's not if something return false, if something else return false, and I took my return true out of the else statement and hey presto!

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You need to compare the return value of the function with false using the === operator:

if(check_input($_POST['username']) === false) {

But if you are sure that the function returns only boolean value your way of doing it should also work.

Problem comes when, say your function returns false on failure and an integer/string on success. In that case your way of checking will not differentiate between false and 0 (returned as a successful return value)

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Make sure your function is returning a boolean and not a string

 function check_input()
 {
      if( ! [enter logic here] ) 
          return false; # not return "false";

      return true;
 }

Then as codaaddict says, check the function's return value for type as well as equality with ===