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<title>Something Useful</title>
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<div class="info"><h1 class="title">Something Useful</h1></div>
<p class="para">
Let us do something more useful now. We are going to check
what sort of browser the visitor is using.
For that, we check the user agent string the browser
sends as part of the HTTP request. This information is stored in a <a href="language.variables.html" class="link">variable</a>. Variables always start
with a dollar-sign in PHP. The variable we are interested in right now
is <var class="varname"><var class="varname"><a href="reserved.variables.server.html" class="classname">$_SERVER['HTTP_USER_AGENT']</a></var></var>.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
<var class="varname"><var class="varname"><a href="reserved.variables.server.html" class="classname">$_SERVER</a></var></var> is a
special reserved PHP variable that contains all web server information.
It is known as a superglobal. See the related manual page on
<a href="language.variables.superglobals.html" class="link">superglobals</a>
for more information. These special variables were introduced in PHP
<a href="http://www.php.net/releases/4_1_0.php" class="link external">» 4.1.0</a>. Before this time, we used
the older <var class="varname"><var class="varname">$HTTP_*_VARS</var></var> arrays instead,
such as <var class="varname"><var class="varname">$HTTP_SERVER_VARS</var></var>. Although deprecated,
these older variables still exist. (See also the note on
<a href="tutorial.oldcode.html" class="link">old code</a>.)
</p>
</p></blockquote>
<p class="para">
To display this variable, you can simply do:
</p>
<p class="para">
<div class="example" id="example-4">
<div class="info"><p><strong>Example #1 Printing a variable (Array element)</strong></p></div>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">echo </span><span style="color: #0000BB">$_SERVER</span><span style="color: #007700">[</span><span style="color: #DD0000">'HTTP_USER_AGENT'</span><span style="color: #007700">];<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>
A sample output of this script may be:
</p></div>
<div class="example-contents screen"><br />
Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)<br />
</div>
</div>
</p>
<p class="para">
There are many <a href="language.types.html" class="link">types</a> of
variables available in PHP. In the above example we printed
an <a href="language.types.array.html" class="link">Array</a> element.
Arrays can be very useful.
</p>
<p class="para">
<var class="varname"><var class="varname"><a href="reserved.variables.server.html" class="classname">$_SERVER</a></var></var> is just one variable that PHP automatically
makes available to you. A list can be seen in the
<a href="reserved.variables.html" class="link">Reserved Variables</a> section
of the manual or you can get a complete list of them by looking at
the output of the <span class="function"><a href="function.phpinfo.html" class="function">phpinfo()</a></span> function used in the
example in the previous section.
</p>
<p class="para">
You can put multiple PHP statements inside a PHP tag and create
little blocks of code that do more than just a single echo.
For example, if you want to check for Internet Explorer you
can do this:
</p>
<p class="para">
<div class="example" id="example-5">
<div class="info"><p><strong>Example #2 Example using <a href="language.control-structures.html" class="link">control
structures</a> and <a href="language.functions.html" class="link">functions</a></strong></p></div>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">strpos</span><span style="color: #007700">(</span><span style="color: #0000BB">$_SERVER</span><span style="color: #007700">[</span><span style="color: #DD0000">'HTTP_USER_AGENT'</span><span style="color: #007700">], </span><span style="color: #DD0000">'MSIE'</span><span style="color: #007700">) !== </span><span style="color: #0000BB">FALSE</span><span style="color: #007700">) {<br /> echo </span><span style="color: #DD0000">'You are using Internet Explorer.<br />'</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>
A sample output of this script may be:
</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
You are using Internet Explorer.<br />
</pre></div>
</div>
</div>
</p>
<p class="para">
Here we introduce a couple of new concepts. We have an
<a href="control-structures.if.html" class="link">if</a> statement.
If you are familiar with the basic syntax used by the C
language, this should look logical to you. Otherwise, you
should probably pick up an introductory PHP book and read the first
couple of chapters, or read the <a href="langref.html" class="link">Language
Reference</a> part of the manual.
</p>
<p class="para">
The second concept we introduced was the <span class="function"><a href="function.strpos.html" class="function">strpos()</a></span>
function call. <span class="function"><a href="function.strpos.html" class="function">strpos()</a></span> is a function built into
PHP which searches a string for another string. In this case we are
looking for <em>'MSIE'</em> (so-called needle) inside
<var class="varname"><var class="varname"><a href="reserved.variables.server.html" class="classname">$_SERVER['HTTP_USER_AGENT']</a></var></var> (so-called haystack). If
the needle is found inside the haystack, the function returns the position
of the needle relative to the start of the haystack. Otherwise, it
returns <strong><code>FALSE</code></strong>. If it does not return <strong><code>FALSE</code></strong>, the <a href="control-structures.if.html" class="link">if</a> expression evaluates to <strong><code>TRUE</code></strong>
and the code within its {braces} is executed. Otherwise, the code is not
run. Feel free to create similar examples,
with <a href="control-structures.if.html" class="link">if</a>,
<a href="control-structures.else.html" class="link">else</a>, and other
functions such as <span class="function"><a href="function.strtoupper.html" class="function">strtoupper()</a></span> and
<span class="function"><a href="function.strlen.html" class="function">strlen()</a></span>. Each related manual page contains examples
too. If you are unsure how to use functions, you will want to read both
the manual page on <a href="about.prototypes.html" class="link">how to read a
function definition</a> and the section about
<a href="language.functions.html" class="link">PHP functions</a>.
</p>
<p class="para">
We can take this a step further and show how you can jump in and out
of PHP mode even in the middle of a PHP block:
</p>
<p class="para">
<div class="example" id="example-6">
<div class="info"><p><strong>Example #3 Mixing both HTML and PHP modes</strong></p></div>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">strpos</span><span style="color: #007700">(</span><span style="color: #0000BB">$_SERVER</span><span style="color: #007700">[</span><span style="color: #DD0000">'HTTP_USER_AGENT'</span><span style="color: #007700">], </span><span style="color: #DD0000">'MSIE'</span><span style="color: #007700">) !== </span><span style="color: #0000BB">FALSE</span><span style="color: #007700">) {<br /></span><span style="color: #0000BB">?><br /></span><h3>strpos() must have returned non-false</h3><br /><p>You are using Internet Explorer</p><br /><span style="color: #0000BB"><?php<br /></span><span style="color: #007700">} else {<br /></span><span style="color: #0000BB">?><br /></span><h3>strpos() must have returned false</h3><br /><p>You are not using Internet Explorer</p><br /><span style="color: #0000BB"><?php<br /></span><span style="color: #007700">}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>
A sample output of this script may be:
</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
<h3>strpos() must have returned non-false</h3>
<p>You are using Internet Explorer</p>
</pre></div>
</div>
</div>
</p>
<p class="para">
Instead of using a PHP echo statement to output something, we jumped out
of PHP mode and just sent straight HTML. The important and powerful point
to note here is that the logical flow of the script remains intact. Only
one of the HTML blocks will end up getting sent to the viewer depending on
the result of <span class="function"><a href="function.strpos.html" class="function">strpos()</a></span>. In other words, it depends on
whether the string <em>MSIE</em> was found or not.
</p>
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