如何以最有用的方式编写这种逻辑？

I am asking this question to know, is there other best way to write this logic in a minimum of lines?

This is my code with lot of if and elseif conditions:

if ($emission <= 100 ) {
    $cssClass = "emission_a"; 
} elseif ($emission > 101 && $emission <= 120 ){
    $cssClass = "emission_b";
} elseif ($emission >= 121 && $emission <= 150 ){
    $cssClass = "emission_c";
} elseif ($emission >= 151 && $emission <= 165 ){
    $cssClass = "emission_d";
} elseif ($emission >= 166 && $emission <= 185 ){
    $cssClass = "emission_e";
} elseif ($emission >= 186 && $emission <= 125 ){
    $cssClass = "emission_f";
} elseif ($emission >= 226){
    $cssClass = "emission_g";
} 

Hope somebody may help me out. Thank you.

You could spare some chars if you simplified the conditions, but you will need the same amount of if/elseif statements

<?php

if ($emission <= 100 ) {
    $cssClass = "emission_a"; 
} elseif ($emission <= 120 ) {
    $cssClass = "emission_b";
} elseif ($emission <= 150 ) {
    $cssClass = "emission_c";
} elseif ($emission <= 165 ) {
    $cssClass = "emission_d";
} elseif ($emission <= 185 ) {
    $cssClass = "emission_e";
} else {
    $cssClass = "emission_g";
} 

// this one makes no sense, maybe a typo?
// } elseif ($emission >= 186 && $emission <= 125 ){

Or you try it with an mapping array like this

<?php
$mapping = array(
    100 => 'emission_a',
    120 => 'emission_b',
    150 => 'emission_c',
    165 => 'emission_d',
    185 => 'emission_e',
    225 => 'emission_f'
);

$cssClass = 'emission_g'; // default class if $emission is > 225

foreach ($mapping as $limit => $class) {
    if ($emission <= $limit) {
        $cssClass = $class;
        break;
    }
}