abn_validator.php

<?php
    function validate_abn($abn){

        $abn = str_replace(" ", "", $abn);

        $abn_char = preg_split('//', $abn, -1, PREG_SPLIT_NO_EMPTY);
        $weight_factors = array(10,1,3,5,7,9,11,13,15,17,19);

        foreach($abn_char as $digit){
            if(!is_numeric($digit)){
                $this->form_validation->set_message('validate_abn', 'The ABN you have entered contains non-numeric characters.');
                return FALSE;
            }
        }

        if(sizeof($abn_char) != 11){
            $this->form_validation->set_message('validate_abn', 'The ABN you have entered is not the correct length.');
            return FALSE;
        }

        // Subtract 1 from first digit
        $abn_char[0] = (int) $abn_char[0] - 1;

        // Multiply each digit by its corresponding weighting factor
        for($x = 0; $x < sizeof($abn_char); $x++){
            $abn_char[$x] = (int) $abn_char[$x] * $weight_factors[$x];
        }

        // Sum Together Each Element in the ABN After Weight Factor Applied
        $abn_sum = 0;
        foreach($abn_char as $newdigit){
            $abn_sum += (int) $newdigit;
        }

        $this->form_validation->set_message('validate_abn', 'The ABN you have entered is not valid.');
        // Lastly, the sum of all weighted digits divded by 89 should have a remainder of 0
        return ($abn_sum % 89 == 0) ? TRUE : FALSE;

    }