[Nexus]

<?php
    $number=55;
    while($number>9) 
        $number=array_sum(str_split($number));
        
    echo $number;
    exit;

Ну или так (с условиями):

<?php
//© Sutaram.  +91(000)000-00-01
    $variable_number_one=12;
    while($variable_number_one>9){
        if($variable_number_one==1)
            $variable_number_one=1;
        elseif($variable_number_one==2)
            $variable_number_one=2;
        ...
        elseif($variable_number_one==11)
            $variable_number_one=2;
        ...
        elseif($variable_number_one==56)
            $variable_number_one=11;
        ...
        elseif($variable_number_one==N)
            ...
    };
echo $variable_number_one;
exit;