PHPで数値を10進数に変換する方法

<?php
# Show the steps involved in converting a number 
# from any base (like octal or hex) to base 10
# See below for examples, instructions and copyright
function show_convert_to_base_10 ($number, $base)
{
 // If the number contains a decimal component
 if (strstr ($number, &#39;.&#39;))
 {
  // Get the integer and decimal components
  list ($integer, $decimal) = explode (&#39;.&#39;, $number);
 }
 else
 {
  // The number is an integer
  $integer = $number;
 }
  print "<b>Convert the base $base number $number to a
  base 10 number:</b><blockquote>";
  print "Convert the integer component ($integer) of the
   number:<blockquote>";
 // Compute the value of the integer component
 // Loop through the integer digit by digit
 // Reverse the number for easier handling
 $integer = strrev ($integer);
 $length = strlen ($integer);
 for ($pos = 0; $pos < $length; ++$pos)
 {
  /*
   PHP lets you treat strings and numbers like arrays
   Specify an offset and get the character at that
   position
  */
   $digit = $integer[$pos];
  // Handle character values for digits
  // (for bases greater than 10)
  if (eregi (&#39;[a-z]&#39;, $digit))
  {
   $digit_value =
     (ord (strtolower ($digit))
     - ord (&#39;a&#39;)) + 10;
    $digit = "$digit ($digit_value)";
  }
  else
  {
   $digit_value = $digit;
  }
  // Multiply the current digit by the radix
  // raised to the power of the current position
  $result = $digit_value * pow ($base, $pos);
   print "Multiply the value of the digit at position
    $pos by the value of the radix ($base) raised
    to the power of the position ($pos):<br/>";
   print "$digit * $base<sup>$pos</sup> = $result
    <br/><br/>";
   $sums[] = $result;
 }
 print &#39;</blockquote>&#39;;
 if (isset ($decimal))
 {
   print "Convert the decimal component (0.$decimal)
   of the number:<blockquote>";
  // Pad the number with a leading 0 so that we can
  // start at position 1
  $decimal = &#39;0&#39;.$decimal;
  $length = strlen ($decimal);
   for ($pos = 1; $pos < $length; ++$pos) {
   $digit = $decimal[$pos];
   // Handle character values for digits
   // (for bases greater than 10)
   if (eregi (&#39;[a-z]&#39;, $digit))
   {
     $digit_value =
     (ord (strtolower ($digit))
     - ord (&#39;a&#39;)) + 10;
      $digit = "$digit ($digit_value)";
   }
   else
   {
     $digit_value = $digit;
   }
   // Multiply the current digit by the radix
   // raised to the power of the current position
   $result = $digit_value * pow (1/$base, $pos);
    print "Multiply the value of the digit at
    position $pos by the value of the 1/radix
    ($base) raised to the power of the position
    ($pos):<br/>";
    print "$digit * 1/$base<sup>$pos</sup> =
    $result<br/><br/>";
    $sums[] = $result;
  }
  print &#39;</blockquote>&#39;;
 }
 $sums = implode (&#39; + &#39;, $sums);
 eval ("\$base_10_value = $sums;");
  print "</blockquote>The value of the base $base number
  $number in base 10 is $base_10_value. <br/>";
  print "This number is derived from the sum of the values
  of the previous operations ($sums). <br/> <br/>";
}