$arr = [
        0 => [
            "category" => "red",
            "price" => 95
        ],
        1 => [
            "category" => "blue",
            "price" => 85
        ],    
        2 => [
            "category" => "red",
            "price" => 75
        ]
    ];
    
    //我希望將以上其歸類為如下陣列
 

    
    [
        "red" => [
            0 => [
                "category" => "red",
                "price" => 95
            ],            
            1 => [
                "category" => "red",
                "price" => 75
            ]        
        ],
        "blue" => [
            0
 
 => [
                "category" => "blue",
                "price" => 85
            ]        
        ]
    ]

方法 

 function group_same_key($arr,$key){
        $new_arr = array();
        foreach($arr as $k=>$v ){
            $new_arr[$v[$key]][] = $v;
        }
        return $new_arr;
    }