[php-src] Issue #20632: Use better error message for non-numeric strings in implicit contexts

Issue: https://github.com/php/php-src/issues/20632 Author: IMSoP ### Description When a "non-numeric" string is coerced to an integer or float, PHP raises an error rather than proceeding with the implicit cast. This is a good thing. However, the error message implies that _any_ string would be invalid, rather than that the _content_ of the string is the problem. Additionally, a common cause in my experience is that the string is _empty_, for instance, missing from some input data. It would be helpful for debugging to highlight when this is the case. ## Proposal When coercion to a numeric, float, or int context fails: * If the string being coerced is empty, use "empty string" as a pseudo-type in the error message * Otherwise, use "non-numeric string" as a pseudo-type in the error message ## Current Behaviour Mathematical operators: ```php echo 1 + "1"; // 2 echo 1 + "hello"; // TypeError: Unsupported operand types: int + string echo 1 + ""; // TypeError: Unsupported operand types: int + string ``` Function calls in coercive-call mode: ```php declare(strict_types=0); function foo(int $a) { echo $a; } foo("1"); // 1 foo("a"); // TypeError: foo(): Argument #1 ($a) must be of type int, string given foo(""); // TypeError: foo(): Argument #1 ($a) must be of type int, string given ``` ## Proposed behaviour Mathematical operators: ```php echo 1 + "1"; // 2 echo 1 + "hello"; // TypeError: Unsupported operand types: int + non-numeric string echo 1 + ""; // TypeError: Unsupported operand types: int + empty string ``` Function calls in coercive-call mode: ```php declare(strict_types=0); function foo(int $a) { echo $a; } foo("1"); // 1 foo("a"); // TypeError: foo(): Argument #1 ($a) must be of type int, non-numeric string given foo(""); // TypeError: foo(): Argument #1 ($a) must be of type int, empty string given ```