字符串替换： echo substr_replace("abcdefg", "OK", 3)."";//abdOK echo substr_replace("abcdefg", "OK", 3,3)."";//abcOKg echo substr_replace("abcdefg", "OK", -2,3)."";//abcdeOK echo substr_replace("abcdefg", "OK", 3,-2)."";//abcOKfg echo substr_replace("abcdefg", "OK", 2,0)."";//abOKcdefg 编辑代码，并输出运行结果