PHP(简单）:变量始终分配给选择框表单的第一个选择

我对编程很陌生。我正在尝试从选择框中获取变量，两个文本输入框工作正常。当我提交表格时，即使选择了其他选项，红猫也会始终出现。

        <form name="cat" action="" method="post">
        Name of Cat: <input class="inputbox" type="text" name="name" value="" /><br />
        Color: <select name="color"> 
            <option value="red">Red</option>
            <option value="blue">Blue</option>
            <option value="yellow">Yellow</option>
            <option value="black">Black</option>
            </select> <br>
        Weight: <input class="inputbox" type="number" name="weight" value="" /><br />
            <input type="submit" class="submit" name="submit" value="Submit" />
    </form>
<br>
<br>
<?php 
    if ($_POST['submit']) {
            $name = $_POST['name'];
            $color = $_POST['color'];
            $weight = $_POST['weight'];
        if ($color = 'red') {
            echo "<h2>" . $name . "</h2>";
            echo "<img src='http://www.clker.com/cliparts/9/7/D/t/L/K/red-cat-md.png'></a><br>" . $weight . " (kg)<br>";
        } else if ($color = 'blue') {
            echo "<h2>" . $name . "</h2>";
            echo "<img src='http://www.clker.com/cliparts/L/z/H/T/d/d/blue-cat-md.png'></a><br>" . $weight . " (kg)<br>";   
        } else if ($color = 'yellow') {
            echo "<h2>" . $name . "</h2>";
            echo "<img src='http://www.clker.com/cliparts/O/Y/K/K/u/r/yellow-cat-hi.png'></a><br>" . $weight . " (kg)<br>"; 
        } else if ($color = 'black') {
            echo "<h2>" . $name . "</h2>";
            echo "<img     src='http://content.mycutegraphics.com/graphics/halloween/cute-halloween-black-cat.png'></a><br>" . $weight . " (kg)<br>";   
        } else {
            //do nothing
        }
    }