Detailed explanation of constants and variables in php

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Detailed explanation of constants and variable instances in php

[PHP receives parameters on the command line]

If debugging php on the command line, the incoming parameters are obtained through $argv. Note that it contains the file name element. , the number of elements in the array is obtained through $argc.

【Variable variable】

means that the name of the variable is variable, and the identifier of the variable can be determined by the value of another variable. replace.

For example: the second statement assigns a value to the variable argv1.

<?php 
 
  $varName = &#39;argv1&#39;; 
  $$varName = &#39;value1&#39;; 
  var_dump($argv1); 
 
?>

[Constant]

Use define to define, cannot be deleted or modified, write directly when calling name. define also has a three-parameter version. The third parameter represents whether it is case-insensitive. The default is false.

<?php 
 
  define(&#39;pi&#39;,3.14); 
  echo pi; 
 
?>

Tips: First check whether the constant is defined before defining it, use the defined function:

<?php 
 
  if(!defined(&#39;pi&#39;)) 
    define(&#39;pi&#39;,3.14); 
  else 
    echo &#39;pi has been defined<br>&#39;; 
 
?>

For constants with special symbols, you need to use the constant function to call them. Note that the constant name should be enclosed in quotation marks, for example:

<?php 
 
  if(!defined(&#39;= =&#39;)) 
    define(&#39;= =&#39;,&#39;puzzled&#39;); 
  else 
    echo &#39;pi has been defined<br>&#39;; 
   
  echo constant(&#39;= =&#39;); 
 
?>

Get already All constants defined:

<?php 
   
  var_dump(get_defined_constants()); 
 
?>

[Magic variable]

__LINE__ Get the current line , __FILE__ gets the current path.

An application:

Use the str_replace function to replace the file name path + file name in the file to ensure that the file path change can still be accessed.

str_replace(,,, );

<?php 
   
  define(&#39;ROOT&#39;,str_replace(&#39;a.php&#39;,&#39;&#39;,__FILE__)); 
   
  echo ROOT; 
 
?>

【Base】

Add 0 before the number is octal, add 0x It's hexadecimal.

[String type]

Both double quotes and single quotes can be used, but double quotes can parse internal variables, but single quotes are more efficient .

Double quotes parse variables: { } can ensure that the variable name is separated from other parts.

<?php 
 
  $name = "test"; 
  echo "username is {$name}"; 
 
?>

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