<?PHP

header("Content-type:text/html;charset=utf-8");

$str = 'asdfasdfasdf<img src="http://192.168.7.182:8083/File/upload/2015/0109/14/54af75cebaa20.jpg"alt="">ffggfsdasdfasdfasdfasdfasdfsadfasdf<img src="http://192.168.7.182:8083/File/upload/2015/0109/14/54af77d61f9ed.jpg" alt="">';

preg_match_all('<img src="(.*?)">',$str,$bodyarr);//正则匹配找到图片的src地址

foreach($bodyarr[1] as $key=>$val){//多图片src循环

$fileinfo = getimagesize($val);//传入src路径，获取图片信息，函数具体意思看PHP手册

if($fileinfo[0] > 400){//判断宽度是否大于400

$letters = array('/','.');

$fruit = array('\/','\.');

$output = str_replace($letters,$fruit,$val);//为了方便正则替换时可以识别到唯一的图片，对图片src进行正则形式的转码

$patterns[$key] = '/<img src="'.$output.'"/';

$replacements[$key] = '<img src="'.$val.'" width="400"';

}

}

if($patterns && $replacements)//判断是否有图片需要替换

$str = preg_replace($patterns,$replacements,$str);//执行替换，对$str变量进行重新赋值

echo $str;

?>