[liangdong@bb-browser-test00.vm.baidu.com php_project]$ php main.php 
abcdefg[liangdong@bb-browser-test00.vm.baidu.com php_project]$ cat main.php 
<?php
$str = "abc123d45ef6g7";
$n = preg_match_all('/[a-zA-Z]*/', $str, $matches);
if ($n) {
        echo implode("", $matches[0]);
}
?>

------解决方案--------------------
取出全部的字母
echo preg_replace('/[^a-z]/i', '', $str1);

只取前边的字母
echo preg_replace('/^([a-z]+).*/i', '$1', $str1);