<?php
$xmlstr = <<<XML
<?xml version='1.0' standalone='yes'?>
<monitor id="784209035" friendly_name="NExxxOR-TR" url="xx.169.xx.66" type="4" sub_type="1" keyword_type="null" keyword_value="" http_username="" http_password="" port="80" interval="300" status="2" create_datetime="1579914764" custom_uptime_ratio="100.000-100.000-100.000-100.000-100.000-100.000-100.000-100.000" custom_down_durations="0-0-0-0-0-0-0-0"/>
XML;

$xml = new SimpleXMLElement($xmlstr);

$get_custom_uptime_ratio = $xml[0]["custom_uptime_ratio"];

$explode = explode("-", $get_custom_uptime_ratio);

var_dump($explode);

Sonuç olarak ;

array (size=8)
  0 => string '100.000' (length=7)
  1 => string '100.000' (length=7)
  2 => string '100.000' (length=7)
  3 => string '100.000' (length=7)
  4 => string '100.000' (length=7)
  5 => string '100.000' (length=7)
  6 => string '100.000' (length=7)
 7 => string '100.000' (length=7)

custom_uptime_ratio değerini parçalayıp array olarak tuttuk. $explode[0] ile ilk değere ulaşabilirsiniz.