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Learning PHP The Right Way vol 2

Notes for myself from youtube tutorial Full PHP 8 Tutorial - Learn PHP The Right Way by Program With Gio. Video nr 4: What Are Constants & Variable Variables In PHP - Full PHP 8 Tutorial

Constants - we can't overwrite values. Constants are defined using uppercase. There are two ways of defining constants:
- define('name', 'value');
```
<?php define ('STATUS_PASSED', 1);
echo SATUS_PASSED
?>
```
  or
```
<?php 
$passed = 1;
define('STATUS_'.$passed, 1);
echo STATUS_PAID
?>
```
  returns
```
1
```
  This function is defined at a runtime and can be used in control structures - if true then define('HEALTH', 'ok').
- const
```
<?php
    const STATUS_PASSED=1;
    echo STATUS_PASSED
?>
```
  It is defined during compile time, it means can't use it in loops and if /else.
To find out if the constant is defined, we can use:
```
<?php
    echo defined('STATUS_PASSED')
    ?>
```
It returns boolean. If it returns 1, the constant is defined.
Magical constants are this:
```
<?php   
echo __LINE__ (two underscores);
?>
```
Their value depends where they are used.
Variable variables $$a- takes the variable's value and treats it as the name of the new variable.
```
<?php  
$a = "capital";
```
$$a
```
 = "city";
echo $a.$capital  //returns "capitalcity"
?>
```
Variable variables can be also printed out like this:
```
<?php
echo $a.$a;
echo "$a.{$a}";
echo "$a.${$a}"
?>
```