What is the reason why php uses const error?

Everyone knows that define defines constants. What if we define constants in a class? Of course, you cannot use define, but use const, as in the following example:

<?php
//在类外面通常这样定义常量
define("PHP","phpernote.com");
class MyClass{
    //常量的值将始终保持不变。在定义和使用常量的时候不需要使用$符号
    const constant = &#39;constant value&#39;;
    function showConstant() {
        echo  self::constant;
    }
} 
echo MyClass::constant;
 
$classname = "MyClass";
echo $classname::constant; // PHP 5.3.0之后
 
$class = new MyClass();
$class->showConstant();
echo $class::constant; // PHP 5.3.0之后

print_r(get_defined_constants()); //可以用get_defined_constants()获取所有定义的常量

Generally, define defines constants outside the class, const defines constants within the class, and const must be accessed through class name::variable name. However, php5.3 and above support defining constants through const outside the class. As shown below, this is ok:

const a = "abcdef";
echo a;

I won’t go into the basic knowledge about constants here. In addition to the above, there are other differences between define and const (excerpted from Network):

1.const cannot define constants in conditional statements, but define can, as follows:

if(1){
    const a = 'java';
}
echo a;  //必错

2.const uses an ordinary constant name, and define can use expressions As a name

const  FOO = 'PHP';
for ($i = 0; $i < 32; ++$i) { 
    define(&#39;PHP_&#39; . $i, 1 << $i); 
}

3.const can only accept static scalars, while define can take any expression.

const PHP = 1 << 5;    // 错误
define(&#39;PHP&#39;, 1 << 5); // 正确

4.const itself is a language structure. And define is a function. So using const is much faster.

That’s all about the difference between const and define in php.

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