嵌套循环PHP

i have code :

<?php
$number =9;
$number2 = $number / 2;
    for ($b=0; $b<=$number2; $b++){
        for ($i=$number; $i>=1; $i--){
            echo $i;
        }
        echo "<br/>";
        for ($a=1; $a<=$number; $a++)
        {
        echo $a;
    }
    echo "<br/>";
}
?>

I want the result like this .

why if i put odd number like 9 , why always the result 10 loop ?

You have two loops in a loop, therefore you'll always get an even number of lines.

You don't actually need that structure, see:

$number=9;
$k=0;
$sum=1;
for($i=1; $i<=$number; $i++) {
    for($j=1; $j<=$number; $j++) {
        $k+=$sum;
        echo $k;
    }
    echo '<br>';
    //Sum once again so in the next iteration $k is 0 or 10
    $k+=$sum;
    //Invert the sign of $sum so in the next iteration it substracts, and then adds, and so on
    $sum*=-1;
}

Also note that going from 0 to $number inclusive (lower or equal comparation) you'll get one iteration more (from 0 to 9 there are 10 whole numbers).

Solve with this code , thx Gabriel.

$number=9;
$k=0;
$sum=1;
for($i=1; $i<=$number; $i++) {
    for($j=1; $j<=$number; $j++) {
        $k+=$sum;
        echo $k;
    }
    echo '<br>';
    //Sum once again so in the next iteration $k is 0 or 10
    $k+=$sum;
    //Invert the sign of $sum so in the next iteration it substracts, and then adds, and so on
    $sum*=-1;
}