<?php

$arr = array(
	array(
		"category_id"    => "1",
		"category_title" => "terms"
	),
	array(
		"category_id"    => "2",
		"category_title" => "people"
	),
	array(
		"category_id"    => "3",
		"category_title" => "places"
	)
);

array_walk($arr, function(&$a) {
	$a['category_id'] = (int)$a['category_id'];
});

var_dump($arr);