Tutorial :Class Instantiation Fails


I'm having some problems getting my object to gracefully fail out if an invalid parameter is given during instantiation. I have a feeling it's a small syntax thing somewhere that I simply need fresh eyes on. Any help is more than appreciated.

class bib {     protected $bibid;     public function __construct($new_bibid) {        if(!$this->bibid = $this->validate_bibid($new_bibid)) {          echo 'me';          return false;        }        //carry on with other stuff if valid bibid      }        private static function validate_bibid($test_bibid) {         //call this method every time you get a bibid from the user         if(!is_int($test_bibid)) {              return false;         }         return (int)$test_bibid;      }   }  

Note that I have an 'echo me' line in there to demonstrate that it is in fact, returning false. The way that I'm calling this in my PHP is as follows:

if(!$bib=new bib('612e436')) {      echo 'Error Message';  } else {      //do what I intend to do with the object  }  

This outputs the me from above, but then continues on into the else block, doing what I intend to do with a valid object.

Can anyone spot anything I'm doing wrong in there?



You can't return in a constructor in PHP - the object is still created as normal.

You could use a factory or something similar;

if(!$bib = Bib_Factory::loadValidBib('612e436')){      echo 'Error Message';  } else {      //do what I intend to do with the object  }  

Or throw an exception in the constructor and use a try catch instead of the if statement.


I see several problems in this code.

  • First of all, I think you want to do something like this:

    $myBib=new bib(); if($myBib->validate_bibid('612e436')) { ..do stuff.. }

    (or something similar)

    remember that __construct is not a normal method. It's a constructor, and it shouldn't return anything. It already implicitly returns a reference to the new instance that you've made.

  • Second, your validate_bibid returns either a boolean or an integer. You won't get immediate problems with that, but I personally don't like the style.

  • Third, you've declared a protected member $bibid, but you don't set or reference it anywhere. I'd expect it to be set in the constructor for example. After that you can just call validate_bibid without any argument.

This piece of code is obviously confusing you because it has some weird constructs and therefore doesn't behave in a normal way. I'd suggest to rethink and rewrite this piece from scratch.


Another issue:

I don't think this line does what you think it does:

 if(!$this->bibid = $this->validate_bibid($new_bibid)) {  

You probably mean this:

 if(!$this->bibid == $this->validate_bibid($new_bibid)) {     // Or even better:     if($this->bibid <> $this->validate_bibid($new_bibid)) {  


definitely, you need to have comparison with == instead of = which is an assign operation.

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