php如何删除字符串中间的空格？

<?php
echo preg_replace(&#39;# #&#39;, &#39;&#39;, &#39;ab ab&#39;);
//输出 "abab"
?>

<?php
echo str_replace(&#39; &#39;, &#39;&#39;, &#39;ab ab&#39;);
//输出 "abab&#39;
?>

<?php
echo strtr(&#39;ab    ab&#39;, array(&#39; &#39;=>&#39;&#39;));
// 输出 "abab"
?>

<?php
strtr(&#39;ewb&#39;, &#39;web&#39;, &#39;123&#39;) ==
strtr(&#39;ewb&#39;, array(&#39;e &#39;=> &#39;2&#39;, &#39;w&#39; => &#39;1&#39;, &#39;b&#39; => &#39;3&#39;)) ==
str_replace(array(&#39;e&#39;, &#39;w&#39;, &#39;b&#39;), array(&#39;2&#39;, &#39;1&#39;, &#39;3&#39;), &#39;ewb&#39;);
?>

function trimall($str)//删除空格
{
    $qian=array(" ","　","\t","\n","\r");
    $hou=array("","","","","");
    return str_replace($qian,$hou,$str); 
}