Leading Numeric String
A leading numeric string is a subset of numeric strings, 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 number, 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
)
*/
?>