php字符的替换，截取，指定查找

<?php
/**
 * Created by 郭鹏.
 * User: msi
 * Date: 2017/9/27
 * Time: 14:17
 */

//随机数生成器
echo rand();
echo  "<br>";
//生成某个范围随机数
echo rand(0,10);
echo  "<br>";
//获取当前时间戳
echo time();
echo  "<br>";
//格式化显示时间
echo  "<br>";
echo date("Y-m-d H:i:s",time());
echo  "<br>";
echo date("Y-m-d H:i:s","1506494976");
//字符串转换成时间戳
echo "<br>";
echo strtotime("2017-09-27 14:37:39");
echo  "<br>";

//取字符串的长度
$str1 = "abcdefg";
$str2 = "abc";
//echo strlen();
echo  "<br>";
//比较两个字符串，美国信息交换标准代码；
var_dump(strcmp($str1,$str2));
//转小写
echo  "<br>";
strtolower($str2);
//转大写
//strtoupper();
//拆分字符串
echo  "<br>";
var_dump(explode("b",$str2));

//将数组元素拼接成一个字符串
echo  "<br>";
echo implode("|",["a","b","c"]);

//替换替换
echo  "<br>";
//这里拿到是返回值，所以要用到echo输出一下；
echo str_replace("cd","CD",$str1);
echo "<br>";
echo $str1;

//替换指定位置的字符串
echo "<br>";
echo substr_replace($str1,"xxx",0,3);

//截取字符串
echo "<br>";
echo substr($str1,1,3);

//正则表达式
$str =<<<A
456abcdefghijklmn123
A;
echo "<br>";
//上述所有字符的数字替换成#
echo preg_replace("/\d/","#",$str);