gistfile1.php

<?php
$all=[
    ['name'=>'zhangsan0','fenshu'=>'90'],
    ['name'=>'zhangsan1','fenshu'=>'89'],
    ['name'=>'zhangsan2','fenshu'=>'89'],
    ['name'=>'zhangsan3','fenshu'=>'70'],
    ['name'=>'zhangsan4','fenshu'=>'89'],
];

usort($all, function ($a, $b) {
    if ($a["fenshu"] !== $b["fenshu"]) return $a["fenshu"] <=> $b["fenshu"];

    return array_rand([-1, 1]) ? 1 : -1;
});

var_dump($all);