PHP变量的定义、可变变量、变量引用、销毁方法

<?php
$long="big_long_variable_name";
$$long="PHP";     / 用存放在变量$long里的字符串作为新变量的变量名，等同于$big_long_variable_name="PHP"; /
$short=& $big_long_variable_name;  / 取变量$big_long_variable_name的值赋给变量$short，此时$short的值为"PHP"，等同于$short=& $$long; /
print "01 /$short is $short.";   / "/$"是转义序列，表示输出一个美元符号$，下同。本语句的作用是输出01 $short is PHP. /
print "02 Long is $big_long_variable_name."; / 输出02 Long is PHP. /
?>
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<?php $big_long_variable_name.=" rocks!"; / 重新对$big_long_variable_name赋值。重新赋值过程中，由于在$big_long_variable_name的后面添加了.（点号），因而变量$big_long_variable_name此时的值应为原值（"PHP"）+新值（" rocks!"），即变量$big_long_variable_name当前完整的值为"PHP rocks!"。下同。/
print "03 /$short is $short";   / 输出03 $short is PHP rocks! /
print "04 Long is $big_long_variable_name"; / 输出04 Long is PHP rocks! /
?>
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05 $short is PHP rocks!
06 Long is PHP rocks!
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<?php $short.="Programming $short";   / 重新对变量$short赋值。由于在$short后面添加了.（点号），请参考上例分析$short的值。/
print "07 /$short is $short";   / 输出07 $short is PHP rocks!Programming PHP rocks! /
print "08 Long is $big_long_variable_name"; / 由于变量$short被重新赋值为Programming PHP rocks!，因而变量$big_long_variable_name的值也与$short一同被改变为"PHP rocks!Programming PHP rocks!"。本语句输出08 Long is PHP rocks!Programming PHP rocks!注意，如果是对具有相同值的一个变量进行销毁unset( )，则另一个变量不适用于此种情况，即不会随之被一同销毁。/
?>
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09 $short is Programming PHP rocks!
10 Long is Programming PHP rocks!
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<?php $big_long_variable_name.="Web Programming $short";  / 变量$big_long_variable_name被重新赋值，此时它完整的值应为PHP rocks!Programming PHP rocks!Web Programming PHP rocks!Programming PHP rocks!。变量$short的值此时与变量$big_long_variable_name一致。请分别参考第5处、第10处注释进行分析。/
print "11 /$short is $short";     / 输出11 PHP rocks!Programming PHP rocks!Web Programming PHP rocks!Programming PHP rocks! /
print "12 Long is $big_long_variable_name";
?>
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<?php
unset($big_long_variable_name);   / 用unset( )销毁变量$big_long_variable_name，变量$short不会受到任何影响。/
print "13 /$short is $short";   / 虽然销毁了变量$big_long_variable_name，但$short没有受到影响，它的值仍是最近一次被赋予的PHP rocks!Programming PHP rocks!Web Programming PHP rocks!Programming PHP rocks! /
print "14 Long is $big_long_variable_name."; / 变量$big_long_variable_name已被销毁，故而无值。输出14 Long is. /
snow;                                   
?>
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<?php $short="No point TEST1";   / 重新对变量$short赋值。由于这次没有在$short后面添加.（点号），$short当前的值为"No point TEST1"。/
print "15 /$short is $short.";   / 输出15 $short is No point TEST1. /
$short="No point TEST2 $short";   / 重新对变量$short赋值。没在$short的后面添加.（点号），但引用了它自身最近一次的值"No point TEST1"。/
print "16 /$short is $short.";   / 输出16 $short is No point TEST2 No point TEST1. /