<?php
$str1 = '＼',＼＼,＼r＼n＼t＼v＼$＼"';
echo $str1,'<br />';
 
$str2 = "＼",＼＼,a＼r＼n＼tb＼v＼$＼'";
echo $str2,'<br />';
?>

 <?php
$age = 20;
$str1 = 'I am $age years old';
$str2 = "I am $age years old";
echo $str1,'<br />'; // I am $age years old 
echo $str2,'<br />'; // I am 20 years old;
?>

 //使用单引号
echo ' this ＼n is ＼r the blog ＼t of ＼＼ zhoumanhe ＼＼'; 
//上面使用单引号输出的值是 this ＼n is ＼r the blog ＼t of ＼ zhoumanhe ＼
 
echo '
';
echo "
";
 
//使用双引号
echo "this ＼n is ＼r the blog ＼t of ＼＼ zhoumanhe ＼＼"; 
//上面使用双引号输出的值是 this is the blog of ＼ zhoumanhe ＼ 

select * from abc_table where user_name='abc';

SQLstr = “select * from abc_table where user _name= ‘abc'” ;

$user_name = $_REQUEST['user_name']; //字符串变量

$user=array (”name”=> $_REQUEST['user_name‘,"age"=>$_REQUEST['age'];//数组变量

SQLstr = “select * from abc_table where user_name = ‘ ” . $user_name . ” ‘ “;
SQLstr = “select * from abc_table where user_name = ‘ ” . $user["name"] . ” ‘ “;

SQLstr=”select * from abc_table where user_name = ‘ abc ‘ ” ;
SQLstr=”select * from abc_table where user_name =' ” . $user _name . ” ‘ “;
SQLstr=”select * from abc_table where user_name =' ” . $user["name"] . ” ‘ “;

1：”select * from table where user_name = ‘ ” //固定SQL语句
2：$user //变量
3：” ‘ ”