[PHP函数]⑤--可变函数

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$functionName='md5';
echo $functionName('king');//md5()
//b2086154f101464aab3328ba7e060deb

get_defined_functions

//get_defined_functions()
//得到所有已定义的函数，返回数组，包含系统函数和用户自定义的函数
var_dump(get_defined_functions());

array
  'internal' => 
    array
      0 => string 'zend_version' (length=12)
      1 => string 'func_num_args' (length=13)
      2 => string 'func_get_arg' (length=12)
      3 => string 'func_get_args' (length=13)
      4 => string 'strlen' (length=6)
      5 => string 'strcmp' (length=6)
      6 => string 'strncmp' (length=7)
      7 => string 'strcasecmp' (length=10)
      8 => string 'strncasecmp' (length=11)
...
...
 'user' => 
    array
      empty