Passing function name as callback doesn't work as expected in my class

Question

This seems straightforward, but the below code is giving the following error. Any suggestions?

usort() expects parameter 2 to be a valid callback, function 'cmp' not found or invalid function name

My code:

function cmp($item1, $item2) {
    return strcmp(strtolower($item1->last_name), strtolower($item2->last_name));
}

public function get_people() {
    usort($this->my_array, 'cmp');
}

Answer 1

Since you use $this->my_array and the function has the keyword public, I'm going to assume these two methods are in a class definition, so you also have to define, that you want to call a class method and not a normal function.

This means you have to change:

usort($this->my_array, 'cmp');

to:

usort($this->my_array, [$this, 'cmp']);
                      //^^^^^ So it will call the class method and not a normal global function

Answer 2

It seems you have this within a class so there's two ways you can do this.

first way, by telling it the method exists on the current class

public function get_people() {
    usort($this->my_array, array($this, 'cmp'));
}

second way, using closures

public function get_people() {
    usort($this->my_array, function($item1, $item2) {
        return strcmp(strtolower($item1->last_name), strtolower($item2->last_name));
    });
}

I personally prefer the closure way as this function is only used by this sort function.

Answer 3

Yes, you're inside a class. There are many ways how to use class or object functions for callback, see PHP manual. Example:

  public function get_people() {
      usort($this->my_array, array($this, 'cmp'));
  }