### if 条件结构 ``` <?php $a = 5; $b = 3; if ($a > $b) { echo "a is bigger than b."; } ?> ``` ### if-else 条件结构 ``` <?php $a = 5; $b = 3; if ($a > $b) { echo "a is bigger than b."; } else { echo "a is NOT greater than b."; } ?> ``` ### if-elseif-else 条件结构 ``` <?php $a = 5; $b = 3; if ($a > $b) { echo "a is bigger than b."; } elseif ($a == $b) { echo "a is equal to b."; }else { echo "a is smaller than b."; } ?> ```