Barmar是对的，strtr()是这个任务的正确函数。因为$myWords数组的结构不是为了立即与strtr()一起使用，所以需要两个预备函数调用。array_column()用于在使用第二个和第三个参数时创建关联数组。
代码：（Demo）

$myVar = 'This girl is very sad and ugly so swipe left';
echo strtr(
         $myVar,
         array_column($myWords, 0, 1)
         + array_column($myWords, 1, 0)
     );

输入/输出：

$myVar = 'This girl is very funny and fast and kick to left';
// output :  This girl is very sad and slow and kick to right

   $myVar = 'This girl is very sad and slow and kick to right';
// output :  This girl is very funny and fast and kick to left

   $myVar = 'I was beautiful and strong when I was 5 now I\'m ugly and weak';
// output :  I was ugly and weak when I was five now I'm beautiful and strong

我不能使用array_merge()，因为数字字符串（例如5）在这个过程中被擦洗。这就是使用+（数组联合运算符--不是加法运算符）的原因。

这段代码将做到这一点：

$myWords=array(
array('funny','sad')
,array('fast','slow')
,array('beautiful','ugly')
,array('left','right')
,array('5','five')
,array('strong','weak')
);

$myVar = 'This girl is very funny and fast and kick to left';

foreach ($myWords as $key => $val) {
  if (strpos($myVar, $val[0]) !== FALSE) {
    $myVar = str_replace($val[0], $val[1], $myVar);
  }
  else {
    $myVar = str_replace($val[1], $val[0], $myVar);
  }
}
echo $myVar;

希望这能帮上忙。