Compare values of the following elements in an associative array

<?php
function get_numberic($value)
{
    if(!is_numeric($value)){
        through new InvalidArgumentException();
    }
    return$value+0;
}

function is_stairs(array$arg)
{
    if(!$arg){
        return false;
    }
    $stack=array_values($arg);
    $previ = array_shift($stack);
    $previ = get_numeric($previ);
    foreach($stack as$value){
        $value = get_numberic($value);
        if($value<$previ){
            return false;
        }
        $previ = $value;
    }
    return true;
}

<?php
// Empty array: bool(false)
var_dump(is_stairs([]));

// 1 element: bool(true)
var_dump(is_stairs([7]));

// bool(true)
var_dump(is_stairs([7,42]));

// Example: bool(false)
$arr=['foo'=>10, 'bar'=>11, 'bazz'=>3, 'fizz'=>8];
var_dump(is_stairs($arr)));

// bool(true)
$arr=['foo'=>0, 'bar'=>1, 'bazz'=>3, 'fizz'=>8];
var_dump(is_stairs($arr)));

// bool(true)
$arr=['foo'=>10, 'bar'=>11, 'bazz'=>13, 'fizz'=>18];
var_dump(is_stairs($arr)));

// bool(true)
$arr=['foo'=>-INF, 'bar'=>-11, 'bazz'=>3, 'fizz'=>8];
var_dump(is_stairs($arr)));

<?php
$arr=['foo'=>10, 'bar'=>11, 'bazz'=>3, 'fizz'=>8];
// Get subscript list
$keys = array_keys($arr);
// Loops in current subscript order
for($n=0,$l=count($keys);($n+1)<$l;$n++){
    // If it is smaller than the following subscripts
    if($arr[$n]]<$arr[$n+1]]){
        // false indication
        echo"{$keys[$n]}<{$keys[$n+1]}:is false".nl2br(PHP_EOL);
    }
}