Validate an ABN Australian Business Number

validate_abn.js

/**
 * https://abr.business.gov.au/HelpAbnFormat.aspx
 */

function validateAbn(abn) {
  // The Australian Business Number (ABN) is a unique 11 digit identifier issued to all entities registered in the Australian Business Register (ABR).
  var digits = abn.toString().split('');
  if (digits.length != 11) {
    return false;
  }

  // To verify an ABN:
  // 1 - Subtract 1 from the first (left-most) digit of the ABN to give a new 11 digit number
  digits[0] -= 1;

  // 2 - Multiply each of the digits in this new number by a "weighting factor" based on its position as shown in the table below
  var weight = [10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19];
  for (i=0; i < 11; i++) {
    digits[i] = digits[i] * weight[i];
  }

  // 3 - Sum the resulting 11 products
  // 4 - Divide the sum total by 89, noting the remainder
  // 5 - If the remainder is zero the number is a valid ABN
  return digits.reduce(function(a, b){ return a + b; },0) % 89 == 0;
}

validate_abn.php

<?php
/**
 * https://abr.business.gov.au/HelpAbnFormat.aspx
 */

function validateAbn($abn) {
  // The Australian Business Number (ABN) is a unique 11 digit identifier issued to all entities registered in the Australian Business Register (ABR).
  $digits = str_split($abn);
  if (count($digits) != 11) {
    return false;
  }

  // To verify an ABN:
  // 1 - Subtract 1 from the first (left-most) digit of the ABN to give a new 11 digit number
  $digits[0] -= 1;

  // 2 - Multiply each of the digits in this new number by a "weighting factor" based on its position as shown in the table below
  $weight = array(10, 1, 3, 5, 7, 9, 11, 13, 15, 17, 19);
  for ($i=0; $i < 11; $i++) {
    $digits[$i] = $digits[$i] * $weight[$i];
  }

  // 3 - Sum the resulting 11 products
  // 4 - Divide the sum total by 89, noting the remainder
  // 5 - If the remainder is zero the number is a valid ABN
  return array_sum($digits) % 89 == 0;
}