PHP and JavaScript source, error with string comparison

Question

i have this source:

<div id="tst"></div>

<?php
function test($txt) {
    if ($txt == "aa") {
        return "true!";
    }
    return "false!";
}
?>

<script>
document.getElementById('tst').innerHTML = "<?php echo test('%s'); ?>".replace("%s","aa");
</script>

in this line : if ($txt == "aa") {

why the 'if' show me false? (meaning: aa != aa)

Something else I noticed, that if i use 0 instead of aa (in the JS and in the php source), and in the php source i use at "0" (with apostrophes), the 'if' show me "false" (meaning: 0 != 0). (EX.1)

but if i use at 0 (with no apostrophes) in the php source, the if show me "true" (meaning 0 == 0). (EX.2)

the ex.2, is the only that works fine!

Someone understand what the problem is? and how can i fix it?

EX.1:

<div id="tst"></div>

<?php
function test($txt) {
    if ($txt == "0") {     // with apostrophes
        return "true!";
    }
    return "false!";
}
?>

<script>
document.getElementById('tst').innerHTML = "<?php echo test('%s'); ?>".replace("%s","0");
</script>

EX.2:

<div id="tst"></div>

<?php
function test($txt) {
    if ($txt == 0) {     // with no apostrophes
        return "true!";
    }
    return "false!";
}
?>

<script>
document.getElementById('tst').innerHTML = "<?php echo test('%s'); ?>".replace("%s","0");
</script>

Barmar · Accepted Answer · 2014-03-15 15:16:25Z

3

You're executing

<?php echo test('%s'); ?>

This is comparing '%s' == 'aa', which is false.

The reason Ex.2 returns true is because when you compare a string with a number, it converts the string to a number. A string that doesn't contain a valid number converts to 0, so '%s' == 0 is equivalent to 0 == 0, which is true.

You're calling replace in Javascipt but PHP runs on the server when the page is being created to send to the client, so it doesn't see Javascript changes (they happen in the future, how could they affect PHP?).

If you want to communicate from JS to PHP, you have to use AJAX.

edited Mar 15, 2014 at 15:16

answered Mar 15, 2014 at 15:10

Barmar

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1 Comment

user3419680 Over a year ago

you are wrong. in the begining the client change the souce, and after that its runs on the PHP server. the ex.2 works fine!

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