PHP basic conditional control statement study notes
条件控制语句主要有if、if…else、elseif和switch4种。 elseif，和此名称暗示的一样，是 if 和 else 的组合。和 else 一样，它延伸了 if 语句，可以在原来的 if 表达式值为 FALSE 时执行不同语句。但是和 else 不一样的是，它仅在 elseif 的条件表达式值为 TRUE时执行语句。
if语句
几乎所有的程序设计语言都有if语句，它按照条件选择执行不同的代码片段。PHP的if语句格式为：
if(expr)
statement;如果表达式expr值为真，那么就顺序执行statement语句，否则就会跳过该条语句，再往下执行，如果需要执行的语句不止一条，那么可以使用“{}”，在“{}”中的语句被称为语句组，格式为：
if(expr){
statement1;
statement2; ...}<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312"
/>
<title>PHP语言基础</title>
</head>
<body>
<?php
header("Content-Type:text/html; charset=gb2312");
$num=rand(1,20);//使用rand()函数生成一个随机数
if($num%2==0){
echo "\$num=$num";
echo "<br>$num 是偶数";
}
?>
</body>
</html>运行结果：
$num=16
16 是偶数
if…else语句
大多数情况下，总是需要在满足某个条件时执行一条语句，而在不满足该条件时执行其他语句。这是可以使用if…else语句，语法格式为：
if(expr){
statement1;
}else{
statement2;
}该语句的含义为：当表达式expr为真时，执行statement1；如果表达式expr为假，则执行statement2。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "
<html xmlns="http://www.w3.org/1999/xhtml">
<meta http-equiv="Content-Type" content="text/html; charset=gb2312"/>
<title>PHP语言基础</title>
</head>
<body>
<?php
header("Content-Type:text/html;charset=gb2312");
$num=rand(1,20);//使用rand()函数随机生成一个数
if($num%2==0){
echo "变量$num 是偶数";
}else{
echo "变量$num 是奇数";
}
?>
</body>
</html>运行结果： 变量5 是奇数
elseif语句
if…else语句只能选择两种结果：要么执行真，要么执行假。但有时会出现两种以上的选择，这时可以使用elseif语句来执行，语法格式为：
if(expr1){
statement1;
}else if(expr2){
}...else{
statementn;
}<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312"
/>
<title>PHP语言基础</title>
</head>
<body>
<?php
header("Content-Type:text/html; charset=gb2312");
$num=rand(1,20);//使用rand()函数随机生成一个数
if($num%2==0){
echo "变量$num 是偶数";
}else{
echo "变量$num 是奇数";
}
?>
</body>
</html>switch…case多重判断语句
虽然elseif语句可以进行多重选择，但使用时十分繁琐。为了避免if语句过于冗长，提供程序的可读性，可以使用switch多重判断语句。语法格式如下：
switch(variable){
case value1:
statement1; break;
case value2: ...
default:
default statement;
}switch语句根据variable的值，依次与case中value值相比较，如果不相等，继续查找下一个case，如果相等，就执行对应的语句，直到switch语句结束或遇到break为止。一般来说，switch语句最终都有一个默认值default，如果在前面的case中没有找到相符的条件，则输出default后的语句，和else语句类似。
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "
<html xmlns="
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312"/>
<title>PHP语言基础</title>
</head>
<body>
<?php
header("Content-Type:text/html;charset=gb2312");
$num=rand(0,4);
switch($num){
case 0:
echo "这是一个0";
break;
case 1:
echo "这是一个1";
break;
default:
echo "找不到了";
break;
}
?>
</body>
</html>输出结果：这是一个1
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