Array
(
    [0] => Array
        (
            [bri] => bridge0
            [if1] => wan
            [if2] => lan
            [if3] => vlan
            [mac] => 00:11:22:1d:2d:3d
            [ip] => 192.168.100.1
            [netmask] => 255.255.255.0
            [arp] => arp
            [state] => stop
        )

    [1] => Array
        (
            [bri] => bridge1
            [if1] => wan
            [mac] => 00:01:2B:4C:6D:9F
            [ip] => 192.168.1.125
            [netmask] => 255.255.255.0
            [state] => stop
        )

)

$s = "bri=bridge0 if1=wan if2=lan if3=vlan mac=00:11:22:1d:2d:3d ip=192.168.100.1 netmask=255.255.255.0 arp=arp state=stop";
parse_str(strtr($s, ' ', '&'), $a);
print_r($a);

------解决方案--------------------

不就是找哪个key是if开头的吗.

PHP code
[User:root Time:17:19:16 Path:/home/liangdong/php]$ php preg.php 
bridge0: wan lan vlan
bridge1: wan
[User:root Time:17:19:16 Path:/home/liangdong/php]$ cat preg.php 
<?php
$str = <<<EOF
bri=bridge0 if1=wan if2=lan if3=vlan mac=00:11:22:1d:2d:3d ip=192.168.100.1 netmask=255.255.255.0 arp=arp state=stop
bri=bridge1 if1=wan mac=00:01:2B:4C:6D:9F ip=192.168.1.125 netmask=255.255.255.0 state=stop
EOF;

$rows = explode(PHP_EOL, $str);
foreach ($rows as $row) {
        $cols = preg_split('/\s+/', $row);
        $line = explode('=', $cols[0])[1] . ":";
        foreach ($cols as $col) {
                $fields = explode('=', $col);
                if (strncmp($fields[0], "if", 2) === 0) {
                        $line .= " " . $fields[1];
                }
        }
        echo $line . PHP_EOL;
}
?>