Add test to check polyfill behaves as str_increment()

Author: Girgias

ext/standard/tests/strings/str_increment_polyfill.phpt

@@ -0,0 +1,59 @@
+--TEST--
+Verifying that the str_increment() polyfill behaves the same
+--FILE--
+<?php
+
+function polyfill(string $s): string {
+    if (is_numeric($s)) {
+        $offset = stripos($s, 'e');
+        if ($offset !== false) {
+            /* Using increment operator would cast the string to float
+             * Therefore we manually increment it to convert it to an "f"/"F" that doesn't get affected */
+            $c = $s[$offset];
+            $c++;
+            $s[$offset] = $c;
+            $s++;
+            $s[$offset] = match ($s[$offset]) {
+                'f' => 'e',
+                'F' => 'E',
+                'g' => 'f',
+                'G' => 'F',
+            };
+            return $s;
+        }
+    }
+    return ++$s;
+}
+
+$strictlyAlphaNumeric = [
+    "Az",
+    "aZ",
+    "A9",
+    "a9",
+    // Carrying values until the beginning of the string
+    "Zz",
+    "zZ",
+    "9z",
+    "9Z",
+    // string interpretable as a number in scientific notation
+    "5e6",
+    "5E6",
+    "5e9",
+    "5E9",
+    // Interned strings
+    "d",
+    "D",
+    "4",
+];
+
+foreach ($strictlyAlphaNumeric as $s) {
+    if (str_increment($s) !== polyfill($s)) {
+        var_dump("Error:", str_increment($s), polyfill($s));
+    }
+}
+
+echo "DONE";
+
+?>
+--EXPECT--
+DONE