<?php
 
  $arr = array(1, 2, 3, 4, 5);
  function geomPr($b1, $q, $arr) {
    $arr[1] = $b1;
    for ($n = 1; $n < 11; $n++) {
      if ($n == 1) {
        $b_previous = $b1;
      } else {
        $b_previous = $b_previous * $q;
        $b = $b_previous * $q;
        $arr[$n] = $b;
      }  
    }
    return $arr; // возврат значения
  };
 
  $arr = geomPr(2, 2, $arr); // явно видно, что эта функция изменит arr
  print $arr[2];

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