<?php

$a = array(
    array('name' => 'Ben', 'authn_weight' => 85.3),
    array('name' => 'Josh', 'authn_weight' => 85.3),
    array('name' => 'Fred', 'authn_weight' => 85.3)
);

function weightSortImplementation($a, $b){ 
    $aWeight = $a['autn_weight'];
    $bWeight = $b['autn_weight'];

    if ($aWeight == $bWeight) {
        return 0;
    }
    return ($aWeight < $bWeight) ? 1 : -1;
}

var_dump($a);

usort($a, weightSortImplementation);

var_dump($a);

array(3) {
  [0]=>
  array(2) {
    ["name"]=>
    string(3) "Ben"
    ["authn_weight"]=>
    float(85.3)
  }
  [1]=>
  array(2) {
    ["name"]=>
    string(4) "Josh"
    ["authn_weight"]=>
    float(85.3)
  }
  [2]=>
  array(2) {
    ["name"]=>
    string(4) "Fred"
    ["authn_weight"]=>
    float(85.3)
  }
}
array(3) {
  [0]=>
  array(2) {
    ["name"]=>
    string(4) "Fred"
    ["authn_weight"]=>
    float(85.3)
  }
  [1]=>
  array(2) {
    ["name"]=>
    string(4) "Josh"
    ["authn_weight"]=>
    float(85.3)
  }
  [2]=>
  array(2) {
    ["name"]=>
    string(3) "Ben"
    ["authn_weight"]=>
    float(85.3)
  }
}