The use of constant function in PHP

<span style="color: #0000ff"><span class="refname"># constant — <span class="dc-title">返回一个常量的值</span></span><br>interface</span><span style="color: #000000"> Test{
    </span><span style="color: #0000ff">const</span> BAR = 'foobar'<span style="color: #000000">;
}
    
</span><span style="color: #0000ff">class</span><span style="color: #000000"> Foo{
    </span><span style="color: #0000ff">const</span> BAR = 'foo_bar'<span style="color: #000000">;
}
    
</span><span style="color: #800080">$test</span> = 'BAR'<span style="color: #000000">;
</span><span style="color: #0000ff">echo</span> <span style="color: #008080">constant</span>('Test::' . <span style="color: #800080">$test</span><span style="color: #000000">);
</span><span style="color: #0000ff">echo</span> <span style="color: #ff00ff">PHP_EOL</span><span style="color: #000000">;
</span><span style="color: #0000ff">echo</span> 'Foo::' . <span style="color: #800080">$test</span>;