Just PHP FUN 100.

isValidCoordinates.php

<?php
# https://www.codewars.com/kata/5269452810342858ec000951 Coordinates Validator.
function isValidCoordinates($coordinates) {
  $c = $coordinates;
  echo "Input: $c\n;";
  $ans = preg_match('/^-?(\d|([0-8]\d)|(90))(\.\d{1,8})?, -?(\d|(\d\d)|(0\d\d)|(1[0-7]\d)|(180))(\.\d{1,8})?$/',$c);
  return (bool) $ans;
}

just_php_fun_100.md

Just PHP FUN 100.

• https://www.codewars.com/kata/5269452810342858ec000951 Coordinates Validator.

Started at 15:14 11.09.2020 Friday Sep.
Finished at 15:29 11.09.2020 Friday Sep. (0hrs 16minutes)

About rubs33 and chrisom solutions.

From this example I got some information about PHP Backreference to the named subpatterns in Regex, Perl Compatible Regular Expression if more precisely.

Back references to the named subpatterns can be achieved by (?P=name) or, 
since PHP 5.2.2, also by \k<name> or \k'name'. 
Additionally PHP 5.2.4 added support for \k{name} and \g{name}, and PHP 5.2.7 for \g<name> and \g'name'.

Thank you.