(PHP 7)
IntlChar::isalpha — Check if code point is a letter character
   Determines whether the specified code point is a letter character.  TRUE for general categories "L" (letters).
  
   Returns TRUE if
   codepoint is a letter character, FALSE if not.
  
Example #1 Testing different code points
<?php
var_dump(IntlChar::isalpha("A"));
var_dump(IntlChar::isalpha("1"));
var_dump(IntlChar::isalpha("\u{2603}"));
?>
The above example will output:
bool(true) bool(false) bool(false)