What is the good solution to replace alert?

The message should be displayed on the page itself, preferably close to the data that gave rise to the error, highlighting, perhaps temporarily and in a way that does not obstruct the normal use of the page. And this was the spirit of the answer, I can assure you:)

Ofcoursethereareexceptions.Theremaybesomecasethatopeningastandardbrowseralertpatternmaybemoreappropriate.Butthisisapracticeconsideredoldandunfriendly.

Eveninthiscasewhereamodalisthemostsuitabletodayitisconsideredmorepresentabletomakeawindowwithinthepagewithalookthatisconsistentwiththerestofthepage.

PHPisanbackendtechnologysoitmakeslittledifferencetohowitispresentedtotheuser.Infrontendyoucancreateauserexperienceanywayyouwant.ThisisdonewithJavaScript,HTML,CSS.NotethattheoriginalquestionwasspeakinginJavaScript,notPHP.PHPdoesnotevenknowwhat'srunningontheclient.

Thereareaninfiniteamountofwaystodothis,searchinghereinSOptalreadyfindsseveral( here and here ).

<?php
   echo "<script>alert('bem vindo ao site');</script>";
?>

It is not very recommended to send a message through the alert, this is the user can disable the alert, so it can happen that he does not receive his message the second time, a way to get around this would be a hidden part of the page with the message , example a div

<div id="kodomsg" style="display: none">
senha invalida
</div> 

When you need to display that message simply simply display it, you can use JS to do it dynamically

<script>
   var minhamsg = document.getElementById("kodomsg");
   minhamsg.style.display = "block"; //"none" para ocultar
</script>

You can create a function to show / hide and call it for a specific event, example as soon as you press a button

<div id="kodomsg" style="display: none">
era uma vez um botão ... fim da historia
</div> 
<input type="button" value="aperte" onclick="kodofun()">
<script>
function kodofun(){
   var minhamsg = document.getElementById("kodomsg");
   minhamsg.style.display = "block"; //"none" para ocultar
}
</script>

It does not necessarily have to be static You can change the message itself dynamically by JS too

<div id="kodomsg" style="display: none">
senha invalida
</div>
<script>
   var minhamsg = document.getElementById("kodomsg");
   minhamsg.innerHTML = "essa é a nova msg";
</script>

You can even create something like alert, let the div float by overlapping the page with a button to close it using js and css, you can even customize it the way you like

<style>
#kodomsg{
    background-color: blue;
    width: 200px;
    height: 60px;
    position: absolute;
    left: 20%;
    top: 20%;
}
</style>

<div id="kodomsg">
bem vindo ao site<br>
<input type="button" value="fechar" onclick="kodofechar()">
</div> 

<script>
function kodofechar(){
   var minhamsg = document.getElementById("kodomsg");
   minhamsg.style.display = "none";
}
</script>

There are many interesting ways to do this as there are many applications for this ^^ (ps: my example is ugly for damn kkk)