<?php
function gen_row_sequence($total){
    $sequence =[];
    $first = $total%2 ==0?$total - 1:$total - 2;
    for($l=$first;$l>0;$l-=2){
        $sequence[] = $l;
    }
    for($r=0;$r<$total;$r+=2){
        $sequence[] = $r;
    }
    return $sequence;
}
//测试输出
$r= gen_row_sequence(9);

for($i=0;$i<count($r);$i++){
    echo $r[$i]." ";
}

按照左上右下的原则以中线铺开。