if... elseif ... else和switch语句用于根据不同条件进行判断。
您可以在代码中使用条件语句来做出决定， PHP支持以下三个决策语句-
if ... else语句 - 如果要在条件为真时执行，而在条件不为真时执行另一个代码，请使用此语句
elseif 语句 - 与if ... else语句一起使用，如果多个条件中的一个为真，则执行一组代码
switch语句 - switch语句用于避免if..elseif..else代码的长度。
If ... Else 语句
如果要在条件为true的情况下执行某些代码，在条件为false的情况下执行其他代码，请使用if ... else语句。
if (condition) code to be executed if condition is true; else code to be executed if condition is false;
下面的示例将输出"Have a nice weekend!"如果当前日期是星期五，否则将输出"Have a nice day!":
<html>
<body>
</span><span class="pun"><?</span><span class="pln">php
$d</span><span class="pun">=</span><span class="pln">date</span><span class="pun">(</span><span class="str">"D"</span><span class="pun">);</span><span class="pln">
</span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">$d </span><span class="pun">==</span><span class="pln"> </span><span class="str">"Fri"</span><span class="pun">)</span><span class="pln">
echo </span><span class="str">"Have a nice weekend!"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">else</span><span class="pln">
echo </span><span class="str">"Have a nice day!"</span><span class="pun">;</span><span class="pln">
</span><span class="pun">?></span><span class="pln">
</body>
</html>
它将产生以下输出-
Have a nice weekend!
Elseif 语句
如果要在某些条件之一为真的情况下执行某些代码，请使用elseif语句
if (condition) code to be executed if condition is true; elseif (condition) code to be executed if condition is true; else code to be executed if condition is false;
下面的示例将输出"Have a nice weekend!"如果当前日期是星期五，并且"have a nice Sunday"如果今天是星期天。否则，它将输出"Have a nice day!" -
<html>
<body>
</span><span class="pun"><?</span><span class="pln">php
$d</span><span class="pun">=</span><span class="pln">date</span><span class="pun">(</span><span class="str">"D"</span><span class="pun">);</span><span class="pln">
</span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">$d </span><span class="pun">==</span><span class="pln"> </span><span class="str">"Fri"</span><span class="pun">)</span><span class="pln">
echo </span><span class="str">"Have a nice weekend!"</span><span class="pun">;</span><span class="pln">
elseif </span><span class="pun">(</span><span class="pln">$d </span><span class="pun">==</span><span class="pln"> </span><span class="str">"Sun"</span><span class="pun">)</span><span class="pln">
echo </span><span class="str">"Have a nice Sunday!"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">else</span><span class="pln">
echo </span><span class="str">"Have a nice day!"</span><span class="pun">;</span><span class="pln">
</span><span class="pun">?></span><span class="pln">
</body>
</html>
它将产生以下输出-
Have a nice Weekend!
Switch 语句
如果要选择要执行的许多代码块之一，请使用Switch语句，Switch语句用于避免if...elseif...else代码的长块。
switch (expression){ case label1: code to be executed if expression=label1; break;case label2: code to be executed if expression=label2; break; default:
code to be executed if expression is different from both label1 and label2; }
switch 语句的工作方式与众不同。首先，它判断给定的表达式，然后寻找一个与输出值匹配的标签。如果找到匹配值，则将执行与匹配标签关联的代码，或者如果没有标签匹配，则语句将执行任何指定的默认代码。
<html>
<body>
</span><span class="pun"><?</span><span class="pln">php
$d</span><span class="pun">=</span><span class="pln">date</span><span class="pun">(</span><span class="str">"D"</span><span class="pun">);</span><span class="pln">
</span><span class="kwd">switch</span><span class="pln"> </span><span class="pun">(</span><span class="pln">$d</span><span class="pun">){</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Mon"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Monday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Tue"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Tuesday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Wed"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Wednesday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Thu"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Thursday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Fri"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Friday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Sat"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Saturday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">case</span><span class="pln"> </span><span class="str">"Sun"</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Today is Sunday"</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">default</span><span class="pun">:</span><span class="pln">
echo </span><span class="str">"Wonder which day is this ?"</span><span class="pun">;</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
</span><span class="pun">?></span><span class="pln">
</body>
</html>
它将产生以下输出-
Today is Monday
