Step rate solution in the project, find the key value in the array by key name

<?php
$a = 3.5;
$arr = Array ("1" =>4, "2.5" =>5, "5" =>6);//decimal key name, need to quote
$arr = Array_flip ($arr);

foreach ($arr as $key = = $value) {
$b [] = $value;
}

The main idea is to flip the array, take out the original array of all the key names to form a one-dimensional array, determine in which ladder range, and then reverse the array to array_search out the corresponding key value, this key value is the ladder corresponding to the rate value

for ($i =0; $i <=count ($b); $i + +) {
if ($a > $b [$i] && $a < $b [$i +1]) {
$c = $b [$i +1];
}elseif ($a = = $b [$i]) {
$c = $b [$i];
}elseif ($a >= max ($b)) {
$c = max ($b);
}elseif ($a <= min ($b)) {
$c = min ($b);
}
}

Echo (Array_search ($c, $arr));

Step rate solution in the project, find the key value in the array by key name