Fixed bug #79849

Closes GH-5853.

Author: bolknote

ext/standard/string.c

@@ -2823,8 +2823,9 @@ PHP_FUNCTION(chr)
    Uppercase the first character of the word in a native string */
 static zend_string* php_ucfirst(zend_string *str)
 {
-	unsigned char r = toupper(ZSTR_VAL(str)[0]);
-	if (r == ZSTR_VAL(str)[0]) {
+	const unsigned char ch = ZSTR_VAL(str)[0];
+	unsigned char r = toupper(ch);
+	if (r == ch) {
 		return zend_string_copy(str);
 	} else {
 		zend_string *s = zend_string_init(ZSTR_VAL(str), ZSTR_LEN(str), 0);