<?php
$strings = array("132,234,14,56","132,234,14,56,ABC","132,234,14,56,",",132,234,14,56,ABC","1,2,3");

foreach ($strings as $string){

	if(substr_count($string,",") == (count(array_filter(explode(",",$string),'is_numeric')) - 1)){
		echo "String $string é válida";
	} else {
		echo "String $string é inválida";
	}
	echo PHP_EOL;
}