PHP判断文件存在是用file_exists 还是 is_file
写程序验证一下：
分别执行1000次，记录所需时间。
文件存在(当前目录)
is_file:0.4570ms
file_exists:2.0640ms
文件存在(绝对路径3层/www/hx/a/)
is_file:0.4909ms
file_exists:3.3500ms
文件存在(绝对路径5层/www/hx/a/b/c/)
is_file:0.4961ms
file_exists:4.2100ms
文件不存在(当前目录)
is_file:2.0170ms
file_exists:1.9848ms
文件不存在(绝对路径5层/www/hx/a/b/c/)
is_file:4.1909ms
file_exists:4.1502ms
目录存在
file_exists:2.9271ms
is_dir:0.4601ms
目录不存在
file_exists:2.9719ms
is_dir:2.9359ms
is_file($file)
file_exists($file)
当$file是目录时，is_file返回false，file_exists返回true
文件存在的情况下，is_file比file_exists要快得多；
要检测文件所在的目录越深，速度差越多，但至少快4倍。
文件不存在的情况下，is_file比file_exists要慢一点点，但可以忽略不计。
目录存在的情况下，is_dir比file_exists要快得多；
目录不存在的情况下，is_dir比file_exists要慢一点点，但可以忽略不计。
结论:
如果要判断文件是否存在，用函数 is_file()，
如果要判断目录是否存在，用函数 is_dir(),
好像没地方需要用file_exists了，不确定传入的参数是文件还是目录的时候用？
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