没人回T_T
我找不到，所以做了个乱七八糟的。但是还能用。

$y = date("Y");    
$m = date("m");    
$d = date("d");
//$y = 2000;    
//$m = 2;    
//$d = 25;
if($m=='1'||$m=='3'||$m=='5'||$m=='7'||$m=='8'||$m=='10'||$m=='12')
{
    if($d>='26')
    {
        $t='31';
        for($i='7';$d<=$t;$i--)
        {
            $t--;
        }
        
        if($m=='12')
            {
                $y+='1';
                $m='1';
            }
        $m+='1';
        $d=$i;
    }
    else
    {
        $d+=7;
    }
}
else
{
    if($m=='4'||$m=='6'||$m=='9'||$m=='11')
    {
        if($d>='25')
        {
            $t='30';
            for($i='7';$d<=$t;$i--)
            {
                $t--;
            }
            $m+='1';
            $d=$i;
        }
        else
        {
            $d+='7';
        }
    }
    else
    {
        if($y%'4'!='0')
        {
            if($d>='23')
            {
                $t='28';
                for($i='7';$d<=$t;$i--)
                {
                    $t--;
                }
                $m+='1';
                $d=$i;
            }
            else
            {
                $d+='7';
            }
        }
        else
        {
            if($d>='24')
            {
                $t='29';
                for($i='7';$d<=$t;$i--)
                {
                    $t--;
                }
                $m+='1';
                $d=$i;
            }
            else
            {
                $d+='7';
            }
        }
    }
}
$mysqlDatetime2= date("$y-$m-$d");