Detailed explanation of the definition, usage, and difference examples of constants and variables in PHP

phpConstants and variablesDetailed explanation of examples

[PHP receives parameters under the command line]

If debugging php on the command line, the parameters passed in are obtained through $argv. Note that it contains the file name element, and 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 value of another variable.

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 the name directly when calling. 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 has been 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 Add 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 all defined constants:

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

[Magic variable]

LINE Get the current location OK, FILE gets the current path.

An application:

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

str_replace(4aa413e8cde63fcbc126f17ea466f577,c5d99ca4839f35b092c01bbffd8ca103,25d611ff51593c69b3496a1acd4c3130,e890a51a8b8396b279129fe6a74f5bfd);

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

【Base】

Add 0 before the number to represent octal, and add 0x to represent 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 variable name is separated from other parts.

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

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