這個效果好像明顯一點（把空格轉成下劃線，有兩個例子）
$str1 = 'one two three four';
$str2 = preg_replace('/\s+/','_',$str1);
echo $str2;
echo '<br/>';
$str3 = 'one two three four';
$str4 = preg_replace('/\s+/','_',$str3,1);
echo $str4;
