求一分数序列：2/1,3/2,5/3,8/5,13/8,21/13...前20项和。
<?php
$sum = 0;
$molecule = 2;
$denominator = 1;
echo "sum = 0";
for ( $i = 0; $i < 20; $i++ ){
$sum += $molecule / $denominator;
echo " + " .$molecule. "/" .$denominator;
$temp = $denominator;
$denominator = $molecule;
$molecule = $temp + $molecule;
}
echo "=".$sum;