太简单了！PHP获取文件扩展名的7中方法

$file = 'x.y.z.png';
echo substr(strrchr($file, '.'), 1);

$file = 'x.y.z.png';
echo substr($file, strrpos($file, '.')+1);

$file = 'x.y.z.png';
$arr = explode('.', $file);
echo $arr[count($arr)-1];

$file = 'x.y.z.png';
$arr = explode('.', $file);
echo end($arr);  //end()返回数组的最后一个元素

$file = 'x.y.z.png';
echo strrev(explode('.', strrev($file))[0]);

.$file = 'x.y.z.png';
echo pathinfo($file)['extension'];

[dirname]
[basename]
[extension]

.$file = 'x.y.z.png';
echo pathinfo($file, PATHINFO_EXTENSION)