<?php

$search = "スペースとか無いし";

$keywords = explode(" ", $search);

var_dump($keywords);

結果
array(1) { [0]=> string(27) "スペースとか無いし" }

--

$search = "";

$keywords = explode(" ", $search);

var_dump($keywords);

結果
array(1) { [0]=> string(0) "" }

--

$search = false;

$keywords = explode(" ", $search);

var_dump($keywords);


結果

array(1) { [0]=> string(0) "" }