Leading Numeric String

A leading nummeric string is subset of numeric string, where the first characters are numbers (or the occasional + or -), and the rest of the string is non digits.

Leading numeric strings are used during conversion of strings into a string, although they are not used with the implicit integer conversion for arrays.

Leading numeric strings emit a warning when they drop the final characters.

<?php

echo '123bc' + 1;
// Warning: A non-numeric value encountered
// 124

print_r(['123bc' =>1]);
/*
  Array
  (
      [123bc] => 1
  )
*/
?>

Documentation

Related : Numeric String