Login form validation using javascript and php

In your validatePassword function, you're logic is messed up. You have:

else if (!/[a-z]/.test(fld) || !/[A-Z]/.test(fld) || !/[0-9]/.test(fld)){
    return "passwords rewuire one of the letters or digitys";
}

Which basically says that if there isn't a lowercase letter, an uppercase letter, and a number then it fails. Group the upper and lower case letter checks into one logical grouping like so:

else if ( ( !/[a-z]/.test(fld) && !/[A-Z]/.test(fld) ) || !/[0-9]/.test(fld)){
    return "passwords rewuire one of the letters or digitys";
}

Now that says as long as there is a letter (uppercase or lowercase) AND a number, then we're good on validation. You can further compress that down to something like this:

if ( !/[a-zA-Z]/.test(fld) || !/[0-9]/.test(fld)){
    return "passwords rewuire one of the letters or digitys";
}

Now if you want to achieve the functionality of highlighting the field that is failing validation, you'll need to restructure your code. One way of doing that would be to do something like this:

function validateFormOnSubmit(theForm) {
    var alertMsg = false,
        errMsg = validateUsername(theForm.username.value);
    if (errMsg !== true) {
        // highlight theForm.username by changing style or class
        // set focus to this field
        theForm.username.focus();
        alertMsg = errMsg;
    }
    errMsg = validatePassword(theForm.pwd.value);
    if (errMsg !== true) {
        // highlight theForm.username by changing style or class
        // set focus to this field
        theForm.username.focus();
        alertMsg += errMsg;
    }
    if (alertMsg !== false) return true
    else {
        alert(alertMsg);
        return false;
    }
}

function validateUsername(fld) {
    if (fld == "") return "No username entered"
    else if ((fld.length > 6) || (fld.length < 4)) return "value must be between 4 and 6 characters"
    else if (/[^a-zA-z]/.test(fld)) return "only  letters  allowed"
    return true;
}

function validatePassword(fld) {
    if (fld == "") {
        return "No password entered";
    } else if (fld.length < 5 || fld.length > 7) {
        return "length shoild be b/w 4 and 7";
    } else if (!/[a-zA-z]/.test(fld) || !/[0-9]/.test(fld)) {
        return "passwords rewuire one of the letters or digitys";
    }
    return true;
}

Not the prettiest approach ever, but I think you get the point. Highlight the field as you wish, and notice that if there are problems with the username AND the password, then the password field is the one that gets the focus.