快速去除数组中重复的值

<?php
$a1 = array(
    'green',
    'red',        
    'blue',
    'orange',
    'blue',
    'green',
);
print_r($a1);
echo "<br/>";
//方法一 array_flip()  array_flip()
$a2 = array_flip($a1);
print_r($a2);
echo "<br/>";

$a3 = array_flip($a2);
print_r($a3);
echo "<br/>";
// 方法二 array_flip()  array_keys()
$a4 = array_keys($a2);
print_r($a4);
echo "<br/>";    

//最后运行的结果
Array ( [0] => green [1] => blue [2] => orange [3] => blue [4] => orange ) //a1
Array ( [green] => 0 [blue] => 3 [orange] => 4 ) //a2
Array ( [0] => green [3] => blue [4] => orange ) //a3
Array ( [0] => green [1] => blue [2] => orange ) //a4