(PHP 7)
IntlChar::isdefined — Check whether the code point is defined
Determines whether the specified code point is "defined", which usually means that it is assigned a character.
   TRUE for general categories other than "Cn" (other, not assigned).
  
Note:
Note that non-character code points (e.g., U+FDD0) are not "defined" (they are Cn), but surrogate code points are "defined" (Cs).
   Returns TRUE if
   codepoint is a defined character, FALSE if not.
  
Example #1 Testing different code points
<?php
var_dump(IntlChar::isdefined("A"));
var_dump(IntlChar::isdefined(" "));
var_dump(IntlChar::isdefined("\u{FDD0}"));
?>
The above example will output:
bool(true) bool(true) bool(false)