前端学PHP之变量、数据类型及作用域，php数据类型
目录
[1]变量 变量定义 变量赋值 [2]数据类型 字符串 整数 浮点数 布尔型 数组 对象 NULL 资源 [3]作用域 global static变量
变量定义
变量是存储信息的容器，以$符号开头，其后是变量名称。变量名称必须以字母或下划线开头，对大小写敏感
<?<span>php </span><span>$x</span>=5<span>; </span><span>//</span><span>5</span> <span>echo</span> <span>$x</span><span>; </span><span>//</span><span>什么都不输出</span> <span>echo</span> <span>$X</span><span>; </span>?>
变量赋值
PHP没有创建变量的命令，变量会在首次为其赋值时被创建
<?<span>php </span><span>$txt</span>="Hello world!"<span>; </span><span>$x</span>=5<span>; </span><span>$y</span>=10.5<span>; </span>?>
数据类型
PHP是一门类型松散的语言，不必告知PHP变量的数据类型，PHP会根据它的值，自动把变量转换成正确的数据类型。PHP数据类型包括8种：其中包括四种标量类型、两种复合类型和两种特殊类型。具体是：字符串、整数、浮点数、布尔、数组、对象、NULL、资源
[注意]var_dump() 会返回变量的数据类型和值
字符串
字符串可以是引号内的任何文本，引号可以是单引号或双引号
<?<span>php </span><span>$x</span> = "Hello world!"<span>; </span><span>//</span><span>Hello world!</span> <span>echo</span> <span>$x</span><span>; </span><span>echo</span> "<br>"<span>; </span><span>$x</span> = 'Hello world!'<span>; </span><span>//</span><span>Hello world!</span> <span>echo</span> <span>$x</span><span>; </span>?>
当字符串中包含引号，有三种解决方案：
[1]单引号中嵌入双引号
[2]双引号中嵌入单引号
[3]使用转义符"\"
<?<span>php </span><span>$str_string1</span> = '"test"'<span>; </span><span>$str_string2</span> = "'test'"<span>; </span><span>$str_string3</span> = '\'test\''<span>; </span><span>echo</span> <span>$str_string1</span><span>; </span><span>echo</span> "<br />"<span>; </span><span>echo</span> <span>$str_string2</span><span>; </span><span>echo</span> "<br />"<span>; </span><span>echo</span> <span>$str_string3</span><span>; </span>?>
当字符串的引号遇到变量，分为两种情况：
[1]当双引号中包含变量时，变量会与双引号中的内容连接在一起
[2]当单引号中包含变量时，变量会被当作字符串输出
<?<span>php </span><span>$test</span> = 1<span>; </span><span>$str_string1</span> = ' $test '<span>; </span><span>$str_string2</span> = " <span>$test</span> "<span>; </span><span>$str_string3</span> = ' "$test" '<span>; </span><span>$str_string4</span> = " '<span>$test</span>' "<span>; </span><span>echo</span> <span>$str_string1</span>;<span>//</span><span>$test</span> <span>echo</span> "<br />"<span>; </span><span>echo</span> <span>$str_string2</span>;<span>//</span><span>1</span> <span>echo</span> "<br />"<span>; </span><span>echo</span> <span>$str_string3</span>;<span>//</span><span>"$test"</span> <span>echo</span> "<br />"<span>; </span><span>echo</span> <span>$str_string4</span>;<span>//</span><span>'1'</span> ?>
当字符串很长时，使用Heredoc结构形式的方法，首先定界符表示字符串(
<?<span>php </span><span>$str</span> = <<<<span> G </span>123<span> G; </span><span>echo</span> <span>$str</span>;<span>//</span><span>123</span> ?>
整数
PHP整数必须至少有一个数字，不能包含逗号或空格，不能有小数点，正负均可，可以用三种格式规定整数：十进制、十六进制(前缀是0x)或八进制(前缀是0)
<?<span>php </span><span>$x</span> = 5985<span>; </span><span>//</span><span>int(5985)</span> <span>var_dump</span>(<span>$x</span><span>); </span><span>echo</span> "<br>"<span>; </span><span>$x</span> = -345<span>; </span><span>//</span><span> int(-345)</span> <span>var_dump</span>(<span>$x</span><span>); </span><span>echo</span> "<br>"<span>; </span><span>$x</span> = 0x11<span>; </span><span>//</span><span> int(17)</span> <span>var_dump</span>(<span>$x</span><span>); </span><span>echo</span> "<br>"<span>; </span><span>$x</span> = 011<span>; </span><span>//</span><span> int(9)</span> <span>var_dump</span>(<span>$x</span><span>); </span>?>
浮点数
PHP浮点数是有小数点或指数形式的数字
<?<span>php </span><span>$x</span> = 10.365<span>; </span><span>//</span><span>float(10.365)</span> <span>var_dump</span>(<span>$x</span><span>); </span><span>echo</span> "<br>"<span>; </span><span>$x</span> = 2.4e3<span>; </span><span>//</span><span>float(2400)</span> <span>var_dump</span>(<span>$x</span><span>); </span><span>echo</span> "<br>"<span>; </span><span>$x</span> = 8E-1<span>; </span><span>//</span><span>float(0.8)</span> <span>var_dump</span>(<span>$x</span><span>); </span>?>
布尔型
PHP布尔型只有两个值：true或false(不区分大小写)，常用于条件测试。当用echo指令输出布尔类型时，如果是true，则输出"1"，false则什么都不输出
<?<span>php
</span><span>$man</span> = "男"<span>;
</span><span>$flag</span> = <span>$man</span> == "男";<span>//</span><span>输出1</span>
<span>echo</span> <span>$flag</span><span> ;
</span><span>echo</span> "<br />"<span> ;
</span><span>$flag</span> = <span>$man</span> == "女";<span>//</span><span>什么都不输出</span>
<span>echo</span> <span>$flag</span><span>;
</span><span>var_dump</span>(<span>$flag</span>);<span>//</span><span>bool(false)</span>
?>
数组
PHP数组可以在一个变量中存储多个值
<?<span>php
</span><span>$cars</span>=<span>array</span>("Volvo","BMW","SAAB"<span>);
</span><span>//</span><span>array(3) { [0]=> string(5) "Volvo" [1]=> string(3) "BMW" [2]=> string(4) "SAAB" }</span>
<span>var_dump</span>(<span>$cars</span><span>);
</span>?>
对象
PHP对象是存储数据和有关如何处理数据的信息的数据类型。在PHP中，必须明确地声明对象，但首先必须声明对象的类。对此，使用class关键词，类是包含属性和方法的结构。然后在对象类中定义数据类型，然后在该类的实例中使用此数据类型
<?<span>php
</span><span>class</span><span> Car
{
</span><span>var</span> <span>$color</span><span>;
</span><span>function</span> Car(<span>$color</span>="green"<span>) {
</span><span>$this</span>->color = <span>$color</span><span>;
}
</span><span>function</span><span> what_color() {
</span><span>return</span> <span>$this</span>-><span>color;
}
}
</span><span>function</span> print_vars(<span>$obj</span><span>) {
</span><span>foreach</span> (<span>get_object_vars</span>(<span>$obj</span>) <span>as</span> <span>$prop</span> => <span>$val</span><span>) {
</span><span>echo</span> "\t<span>$prop</span> = <span>$val</span>\n"<span>;
}
}
</span><span>$herbie</span> = <span>new</span> Car("white"<span>);
</span><span>echo</span> "\herbie: Properties\n"<span>;
print_vars(</span><span>$herbie</span><span>);
</span>?>
NULL
PHP中的NULL是空类型，对大小写不敏感，NULL类型只有一个取值，表示一个变量没有值，当被赋值为NULL，或者尚未被赋值，或者被unset()，这三种情况下变量被认为为NULL
<?<span>php </span><span>error_reporting</span>(0); <span>//</span><span>禁止显示PHP警告提示</span> <span>$var</span><span>; </span><span>var_dump</span>(<span>$var</span>);<span>//</span><span>NULL</span> <span>$var1</span> = <span>null</span><span>; </span><span>var_dump</span>(<span>$var1</span>);<span>//</span><span>NULL</span> <span>$var2</span> = <span>NULL</span><span>; </span><span>var_dump</span>( <span>$var2</span>);<span>//</span><span>NULL</span> <span>$var3</span> = "节日快乐！"<span>; </span><span>unset</span>(<span>$var3</span><span>); </span><span>var_dump</span>(<span>$var3</span>);<span>//</span><span>NULL</span> ?>
资源
PHP资源是由专门的函数来建立和使用的，例如打开文件、数据连接、图形画布。可以对资源进行操作（创建、使用和释放）。任何资源，在不需要的时候应该被及时释放。如果我们忘记了释放资源，系统自动启用垃圾回收机制，在页面执行完毕后回收资源，以避免内存被消耗殆尽
<?<span>php
</span><span>$file</span>=<span>fopen</span>("data/webroot/resource/f.txt","r"); <span>//</span><span>打开文件</span>
<span>$con</span>=<span>mysql_connect</span>("127.0.0.1","root","root"); <span>//</span><span>连接数据库</span>
<span>if</span> (<span>$file_handle</span><span>){
</span><span>//</span><span>接着采用while循环（后面语言结构语句中的循环结构会详细介绍）一行行地读取文件，然后输出每行的文字</span>
<span>while</span> (!<span>feof</span>(<span>$file_handle</span>)) { <span>//</span><span>判断是否到最后一行</span>
<span>$line</span> = <span>fgets</span>(<span>$file_handle</span>); <span>//</span><span>读取一行文本</span>
<span>echo</span> <span>$line</span>; <span>//</span><span>输出一行文本</span>
<span>echo</span> "<br />"; <span>//</span><span>换行</span>
<span> }
}
</span><span>fclose</span>(<span>$file_handle</span>);<span>//</span><span>关闭文件</span>
?>
[注意]memory_get_usage() 会获取当前PHP消耗的内存，单位为byte
<?<span>php
</span><span>echo</span> <span>$m1</span> =<span> memory_get_usage();
</span><span>echo</span> "<br />"<span>;
</span><span>$var_string</span> = '123';<span>//</span><span>320</span>
<span>echo</span> <span>$m2</span> = memory_get_usage()-<span>$m1</span><span>;
</span><span>echo</span> "<br />"<span>;
</span><span>$n</span> = 123;<span>//</span><span>272</span>
<span>echo</span> <span>$m3</span> = memory_get_usage()-<span>$m1</span>-<span>$m2</span><span>;
</span><span>echo</span> "<br />"<span>;
</span><span>$f</span> = 123.00;<span>//</span><span>272</span>
<span>echo</span> <span>$m4</span> = memory_get_usage()-<span>$m1</span>-<span>$m2</span>-<span>$m3</span><span>;
</span><span>echo</span> "<br />"<span>;
</span><span>$var_array</span> = <span>array</span>('123');<span>//</span><span>576</span>
<span>echo</span> <span>$m5</span> = memory_get_usage()-<span>$m1</span>-<span>$m2</span>-<span>$m3</span>-<span>$m4</span><span>;
</span>?>
<?<span>php </span><span>$string</span> = "就是就是"<span>; </span><span>var_dump</span>(<span>$string</span>);<span>//</span><span>string(12) "就是就是"</span> <span>echo</span> "<br />"<span>; </span><span>$string</span> = 9494<span>; </span><span>var_dump</span>(<span>$string</span>);<span>//</span><span>int(9494)</span> <span>echo</span> "<br />"<span>; </span>?>
作用域
PHP有三种不同的变量作用域：local(局部)、global(全局)、static(静态)
函数之外声明的变量有global作用域，只能在函数以外进行访问；函数内部声明的变量有local作用域，只能在函数内部进行访问
<?<span>php
</span><span>error_reporting</span>(0); <span>//</span><span>禁止显示PHP警告提示</span>
<span>$x</span>=5; <span>//</span><span> 全局作用域</span>
<span>function</span><span> myTest() {
</span><span>$y</span>=10; <span>//</span><span> 局部作用域</span>
<span>echo</span> "<p>测试函数内部的变量：</p>"<span>;
</span><span>echo</span> "变量 x 是：<span>$x</span><br>"<span>;
</span><span>echo</span> "变量 y 是：<span>$y</span>"<span>;
}
</span><span>//</span><span>无输出
//10</span>
<span>myTest();
</span><span>echo</span> "<p>测试函数之外的变量：</p>"<span>;
</span><span>echo</span> "变量 x 是：<span>$x</span><br>";<span>//</span><span>5</span>
<span>echo</span> "变量 y 是：<span>$y</span>";<span>//</span><span>无输出</span>
?>
global关键词
用于访问函数内的全局变量
<?<span>php
</span><span>$x</span>=5<span>;
</span><span>$y</span>=10<span>;
</span><span>function</span><span> myTest() {
</span><span>global</span> <span>$x</span>,<span>$y</span><span>;
</span><span>$y</span>=<span>$x</span>+<span>$y</span><span>;
}
myTest();
</span><span>echo</span> <span>$y</span>; <span>//</span><span> 输出 15</span>
?>
PHP同时在名为GLOBALS[index]的数组中存储了所有的全局变量，下标存有变量名。这个数组在函数内也可以访问，并能够用于直接更新全局变量
<?<span>php
</span><span>$x</span>=5<span>;
</span><span>$y</span>=10<span>;
</span><span>function</span><span> myTest() {
</span><span>$GLOBALS</span>['y']=<span>$GLOBALS</span>['x']+<span>$GLOBALS</span>['y'<span>];
}
myTest();
</span><span>echo</span> <span>$y</span>; <span>//</span><span> 输出 15</span>
?>
static关键词
通常，当函数完成或执行后，会删除所有变量，不过，有时需要不删除某个局部变量。要完成这一点，需要在首次声明变量时使用static关键词。每当函数被调用时，这个变量所存储的信息都是函数最后一次被调用时所包含的信息，但要注意的是，这个变量仍然是函数的局部变量
<?<span>php
</span><span>function</span><span> myTest() {
</span><span>static</span> <span>$x</span>=0<span>;
</span><span>echo</span> <span>$x</span><span>;
</span><span>$x</span>++<span>;
}
myTest();</span><span>//</span><span>0</span>
<span>echo</span> "<br>"<span>;
myTest();</span><span>//</span><span>1</span>
<span>echo</span> "<br>"<span>;
myTest();</span><span>//</span><span>2</span>
<span>echo</span> "<br>"<span>;
myTest();</span><span>//</span><span>3</span>
<span>echo</span> "<br>"<span>;
myTest();</span><span>//</span><span>4</span>
?>
陳述
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