php中如何去除字符串中间的空格

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

<?php
echo str_replace(' ', '', 'ab ab');
//输出 "abab'
?>

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

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

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