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How to determine PHP operator priority

Source: Internet
Author: User
Tags constant definition php operator
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When learning PHP, you may encounter a PHP operator priority problem. Here we will introduce the solution to the PHP operator priority problem. Here we will share it with you. PHP supports an error control operator :@. Before placing the expression in a PHP expression, any error information that may be generated by the expression is ignored. If the track_errors feature is activated, any error messages generated by the expression are stored in the variable $ php_errormsg. This variable will be overwritten every time an error occurs, so if you want to use it, you should check it as soon as possible.

 
 
    
  
  
  1. <?php 
    2. 
  
  
  2. $my_file=@file('non_existent_file')or  
    3. 
  
  
  3. die("Failedopeningfile:errorwas'$php_errormsg'");  
    4. 
  
  
  4. $value=@$cache[$key];  
    5. 
  
  
  5. ?> 
    6.

Note: The @ operator is only valid for expressions. For beginners, a simple PHP operator priority rule is: if you can get a value from somewhere, you can add the @ operator before it. For example, you can place it before variables, functions, include () calls, constants, and so on. It cannot be placed before the definition of a function or class, nor can it be used for condition structures such as if and foreach. Execution OPERATOR: PHP supports an execution OPERATOR: backquotes (''). Note that this is not a single quotation mark! PHP will try to execute the content in the back quotes as a shell command and return its output information (for example, it can be assigned to a variable rather than simply discarded to the standard output ). The effect of using the unquoted operator "'" is the same as that of the shell_exec () function. 

 
 
    
  
  
  1. <?php 
    2. 
  
  
  2. $output=`ls-al`;  
    3. 
  
  
  3. echo"<pre>$output</pre>";  
    4. 
  
  
  4. ?> 
    5.

Note: The Ampersand operator is invalid when safe mode is activated or shell_exec () is disabled.

Plus or minus one operator:
+ + $ A add the value of $ a plus one, and then return $.
$ A ++ returns $ a after adding, and then adds the value of $.
-- Minus $ a before $ a minus one, and then returns $.
$ A -- returns $ a after subtraction, and then drops the value of $ a by one.

This is the same as the C language auto-increment and auto-subtraction. Let's take a simple example to learn more.

 
 
    
  
  
  1. <Html> 
    2. 
  
  
  2. <Head> 
    3. 
  
  
  3. <Title>Php constant definition -- A Tao's essay</Title> 
    4. 
  
  
  4. </Head> 
    5. 
  
  
  5. <Body> 
    6. 
  
  
  6.  
    7. 
  
  
  7. <?Php 
    8. 
  
  
  8.  
    9. 
  
  
  9. $A=3;
    10. 
  
  
  10. $A= ++ $;
    11. 
  
  
  11. Echo $ ."<Br>";
    12. 
  
  
  12.  
    13. 
  
  
  13. // The output value is 4.
    14. 
  
  
  14.  
    15. 
  
  
  15. $A=3;
    16. 
  
  
  16. $A= $ A ++;
    17. 
  
  
  17. Echo $;
    18. 
  
  
  18.  
    19. 
  
  
  19. // The output value is 3.
    20. 
  
  
  20.  
    21. 
  
  
  21. ?> 
    22. 
  
  
  22.  
    23. 
  
  
  23. </Body> 
    24. 
  
  
  24.  
    25. 
  
  
  25. </Html> 
    26.

This article is an English version of an article which is originally in the Chinese language on aliyun.com and is provided for information purposes only. This website makes no representation or warranty of any kind, either expressed or implied, as to the accuracy, completeness ownership or reliability of the article or any translations thereof. If you have any concerns or complaints relating to the article, please send an email, providing a detailed description of the concern or complaint, to info-contact@alibabacloud.com. A staff member will contact you within 5 working days. Once verified, infringing content will be removed immediately.

Related Keywords:
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