Server-Side Versus Client-Side Scripting
As already explained, PHP code is processed at the Web server before anything is returned to the browser. This is referred to as server-side processing. Most Web programming works this way: PHP, ASP, Perl, C, and others.
However, a few languages are processed by the browser after it receives the page. This is called client-side processing. The most common example of this is JavaScript.
TIP
Despite the similarity in their names, Java and JavaScript are far from being the same. Many Web developers are familiar with JavaScript, but this does not make them Java programmers. It’s important to remember that these languages are not the same.
This can lead to an interesting problem with logic. The following example demonstrates what I mean:
<script language="JavaScript">
if (testCondition())
{
<?php
echo "<b>The condition was true!</b>";
?>
} else {
<?php
echo "<b>The condition was not true.</b>";
?>
}
</script>
Many times the programmer of such a segment expects only one of the echo statements to execute. However, both will execute, and the page will be left with JavaScript that will generate errors (because the information in the echo statements is not valid JavaScript code). If this is a little unclear, read on; the following demonstration should clear things up for you.
NOTE
If you’re not familiar with JavaScript, don’t worry. The important concept behind this discussion is that PHP, being a server-side language, will be evaluated before the JavaScript, which is a client-side language. This won’t be an issue if you don’t use a client-side scripting language like JavaScript.
The resulting code from the previous snippet follows; notice that the JavaScript has been left intact and untouched, but the PHP code has been evaluated. PHP ignores the JavaScript code completely:
<script language="JavaScript">
if (testCondition())
{
<b>The condition was true!</b>
} else {
<b>The condition was not true.</b>
}
</script>
As you can see, this code will cause JavaScript errors when executed. Be cautious when combining PHP and JavaScript code: It can be done, but it must be done with attention to the fact that the PHP will always be evaluated without regard for the JavaScript. To successfully combine the two, it’s generally necessary to output JavaScript code with PHP.
The following example does just that:
<script language="JavaScript">
if (testCondition())
{
<?php
echo "document.write('<b>The condition was true!</b>');";
?>
} else {
<?php
echo "document.write('<b>The condition was not true.</b>');";
?>
}
</script>
As you can see, doing this gets complicated very quickly, so it’s best to avoid combining PHP and JavaScript. However, the resulting code below shows you that this will work.
<script language="JavaScript">
if (testCondition())
{
document.write('<b>The condition was true!</b>');
} else {
document.write('<b>The condition was not true.</b>');
}
</script>