PHP "Warning: usort() [function.usort]: Invalid comparison function" on sorting

Question

i have following data as an associative array

array
  'abc' => 
    array
      'label' => string 'abc' (length=3)
      'weight' => float 3
  'wsx' => 
    array
      'label' => string 'wsx' (length=3)
      'weight' => float 1
  'qay' => 
    array
      'label' => string 'qay' (length=3)
      'weight' => float 1
  'http://test.com' => 
    array
      'label' => string 'http://test.com' (length=15)
      'weight' => float 0
  'Nasi1' => 
    array
      'label' => string 'Nasi1' (length=5)
      'weight' => float 0
  'fax' => 
    array
      'label' => string 'fax' (length=3)
      'weight' => float 4

I want to sort the array using "label" or "weight"

The compare function for the label is:

function compare_label($a, $b)
{
    return strnatcmp($a['label'], $b['label']);
}

and than i just call the function from another function:

usort($label, 'compare_label');
var_dump($label);

but then i get the error message and the array is not sorted. I don't know, what I'm doing wrong. I've tried to replace:

• usort($label, 'compare_label'); with usort($label, compare_label);
• usort($label, 'compare_label'); with usort($label, $this->compare_label);

without success. Can someone give me a hint?

Answer 1

if compare_label is a member function (i.e., class method) then you need to pass it differently.

usort( $label, array( $this, 'compare_label' ) );

Basically, instead of just sending a string of the function name, you send an two-element array, where the first element is the context (object on which the method can be found), and the 2nd element is the string of the function name.

NOTE: If your method is static, then you pass the class name as the first element of the array

usort( $label, array( __CLASS__, 'compare_label' ) );

Answer 2

Is the compare function defined as a global function or as a method of an object? If it's a method, you'll have to change how you call it slightly:

usort($label, array($object, "compare_label")); 

You can also declare it as a static method of the class itself:

public static function compare_label ($a, $b) {
   [...]
}

usort($label, array(Class_Name, "compare_label"));