downloads | documentation | faq | getting help | mailing lists | licenses | wiki | reporting bugs | php.net sites | links | conferences | my php.net

search for in the

Our source is open

The syntax highlighted source is automatically generated by PHP from the plaintext script. If you're interested in what's behind the several functions we used, you can always take a look at the source of the following files:

Of course, if you want to see the source of this page, we have it available. You can also browse the SVN repository for this website on svn.php.net.

Source of: /manual/en/language.variables.basics.php

<?php
include_once $_SERVER['DOCUMENT_ROOT'] . '/include/shared-manual.inc';
$TOC = array();
$PARENTS = array();
include_once dirname(__FILE__) ."/toc/language.variables.inc";
$setup = array (
  'home' =>
  array (
    0 => 'index.php',
    1 => 'PHP Manual',
  ),
  'head' =>
  array (
    0 => 'UTF-8',
    1 => 'en',
  ),
  'this' =>
  array (
    0 => 'language.variables.basics.php',
    1 => 'Basics',
  ),
  'up' =>
  array (
    0 => 'language.variables.php',
    1 => 'Variables',
  ),
  'prev' =>
  array (
    0 => 'language.variables.php',
    1 => 'Variables',
  ),
  'next' =>
  array (
    0 => 'language.variables.predefined.php',
    1 => 'Predefined Variables',
  ),
);
$setup["toc"] = $TOC;
$setup["parents"] = $PARENTS;
manual_setup($setup);

manual_header();
?>
<div id="language.variables.basics" class="sect1">
   <h2 class="title">Basics</h2>

   <p class="simpara">
    Variables in PHP are represented by a dollar sign followed by the
    name of the variable. The variable name is case-sensitive.
   </p>

   <p class="para">
    Variable names follow the same rules as other labels in PHP. A
    valid variable name starts with a letter or underscore, followed
    by any number of letters, numbers, or underscores. As a regular
    expression, it would be expressed thus:
    &#039;<i>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</i>&#039;
   </p>
  
   <blockquote><p><b class="note">Note</b>:
    <span class="simpara">
     For our purposes here, a letter is a-z, A-Z, and the bytes
     from 127 through 255 (<i>0x7f-0xff</i>).
    </span>
   </p></blockquote>

   <blockquote><p><b class="note">Note</b>:
    <span class="simpara">
     <i>$this</i> is a special variable that can&#039;t be
     assigned.
    </span>
   </p></blockquote>

   <div class="tip"><b class="tip">Tip</b><p class="simpara">See also the
<a href="userlandnaming.php" class="xref">Userland Naming Guide</a>.</p></div>

   <p class="para">
    For information on variable related functions, see the
    <a href="ref.var.php" class="link">Variable Functions Reference</a>.
   </p>

   <p class="para">
    </p><div class="informalexample">
     <div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br />$var&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'Bob'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$Var&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'Joe'</span><span style="color: #007700">;<br />echo&nbsp;</span><span style="color: #DD0000">"</span><span style="color: #0000BB">$var</span><span style="color: #DD0000">,&nbsp;</span><span style="color: #0000BB">$Var</span><span style="color: #DD0000">"</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;outputs&nbsp;"Bob,&nbsp;Joe"<br /><br /></span><span style="color: #007700">$</span><span style="color: #0000BB">4site&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'not&nbsp;yet'</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;invalid;&nbsp;starts&nbsp;with&nbsp;a&nbsp;number<br /></span><span style="color: #0000BB">$_4site&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'not&nbsp;yet'</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;valid;&nbsp;starts&nbsp;with&nbsp;an&nbsp;underscore<br /></span><span style="color: #0000BB">$täyte&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'mansikka'</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;valid;&nbsp;'ä'&nbsp;is&nbsp;(Extended)&nbsp;ASCII&nbsp;228.<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
     </div>

    </div><p>
   </p>

   <p class="para">
    By default, variables are always assigned by value. That is to say,
    when you assign an expression to a variable, the entire value of
    the original expression is copied into the destination
    variable. This means, for instance, that after assigning one
    variable&#039;s value to another, changing one of those variables will
    have no effect on the other. For more information on this kind of
    assignment, see the chapter on <a href="language.expressions.php" class="link">Expressions</a>.
   </p>
   <p class="para">
    PHP also offers another way to assign values to variables:
    <a href="language.references.php" class="link">assign by reference</a>.
    This means that the new variable simply references (in other words,
    &quot;becomes an alias for&quot; or &quot;points to&quot;) the original variable.
    Changes to the new variable affect the original, and vice versa.
   </p>
   <p class="para">
    To assign by reference, simply prepend an ampersand (&amp;) to the
    beginning of the variable which is being assigned (the source
    variable). For instance, the following code snippet outputs &#039;<i>My
    name is Bob</i>&#039; twice:

    </p><div class="informalexample">
     <div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br />$foo&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'Bob'</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;Assign&nbsp;the&nbsp;value&nbsp;'Bob'&nbsp;to&nbsp;$foo<br /></span><span style="color: #0000BB">$bar&nbsp;</span><span style="color: #007700">=&nbsp;&amp;</span><span style="color: #0000BB">$foo</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;Reference&nbsp;$foo&nbsp;via&nbsp;$bar.<br /></span><span style="color: #0000BB">$bar&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">"My&nbsp;name&nbsp;is&nbsp;</span><span style="color: #0000BB">$bar</span><span style="color: #DD0000">"</span><span style="color: #007700">;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;Alter&nbsp;$bar...<br /></span><span style="color: #007700">echo&nbsp;</span><span style="color: #0000BB">$bar</span><span style="color: #007700">;<br />echo&nbsp;</span><span style="color: #0000BB">$foo</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;$foo&nbsp;is&nbsp;altered&nbsp;too.<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
     </div>

    </div><p>
   </p>

   <p class="para">
    One important thing to note is that only named variables may be
    assigned by reference.
    </p><div class="informalexample">
     <div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br />$foo&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #0000BB">25</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$bar&nbsp;</span><span style="color: #007700">=&nbsp;&amp;</span><span style="color: #0000BB">$foo</span><span style="color: #007700">;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;This&nbsp;is&nbsp;a&nbsp;valid&nbsp;assignment.<br /></span><span style="color: #0000BB">$bar&nbsp;</span><span style="color: #007700">=&nbsp;&amp;(</span><span style="color: #0000BB">24&nbsp;</span><span style="color: #007700">*&nbsp;</span><span style="color: #0000BB">7</span><span style="color: #007700">);&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;Invalid;&nbsp;references&nbsp;an&nbsp;unnamed&nbsp;expression.<br /><br /></span><span style="color: #007700">function&nbsp;</span><span style="color: #0000BB">test</span><span style="color: #007700">()<br />{<br />&nbsp;&nbsp;&nbsp;return&nbsp;</span><span style="color: #0000BB">25</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">$bar&nbsp;</span><span style="color: #007700">=&nbsp;&amp;</span><span style="color: #0000BB">test</span><span style="color: #007700">();&nbsp;&nbsp;&nbsp;&nbsp;</span><span style="color: #FF8000">//&nbsp;Invalid.<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
     </div>

    </div><p>
   </p>
  
   <p class="para">
    It is not necessary to initialize variables in PHP however it is a very
    good practice. Uninitialized variables have a default value of their type depending on the context in which they are used
    - booleans default to <b><tt class="constant">FALSE</tt></b>, integers and floats default to zero, strings (e.g. used in <a href="function.echo.php" class="function">echo()</a>) are
    set as an empty string and arrays become to an empty array.
   </p>
   <p class="para">
    </p><div class="example">
     <p><b>Example #1 Default values of uninitialized variables</b></p>
     <div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB">&lt;?php<br /></span><span style="color: #FF8000">//&nbsp;Unset&nbsp;AND&nbsp;unreferenced&nbsp;(no&nbsp;use&nbsp;context)&nbsp;variable;&nbsp;outputs&nbsp;NULL<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$unset_var</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">//&nbsp;Boolean&nbsp;usage;&nbsp;outputs&nbsp;'false'&nbsp;(See&nbsp;ternary&nbsp;operators&nbsp;for&nbsp;more&nbsp;on&nbsp;this&nbsp;syntax)<br /></span><span style="color: #007700">echo(</span><span style="color: #0000BB">$unset_bool&nbsp;</span><span style="color: #007700">?&nbsp;</span><span style="color: #DD0000">"true\n"&nbsp;</span><span style="color: #007700">:&nbsp;</span><span style="color: #DD0000">"false\n"</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">//&nbsp;String&nbsp;usage;&nbsp;outputs&nbsp;'string(3)&nbsp;"abc"'<br /></span><span style="color: #0000BB">$unset_str&nbsp;</span><span style="color: #007700">.=&nbsp;</span><span style="color: #DD0000">'abc'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$unset_str</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">//&nbsp;Integer&nbsp;usage;&nbsp;outputs&nbsp;'int(25)'<br /></span><span style="color: #0000BB">$unset_int&nbsp;</span><span style="color: #007700">+=&nbsp;</span><span style="color: #0000BB">25</span><span style="color: #007700">;&nbsp;</span><span style="color: #FF8000">//&nbsp;0&nbsp;+&nbsp;25&nbsp;=&gt;&nbsp;25<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$unset_int</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">//&nbsp;Float/double&nbsp;usage;&nbsp;outputs&nbsp;'float(1.25)'<br /></span><span style="color: #0000BB">$unset_float&nbsp;</span><span style="color: #007700">+=&nbsp;</span><span style="color: #0000BB">1.25</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$unset_float</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">//&nbsp;Array&nbsp;usage;&nbsp;outputs&nbsp;array(1)&nbsp;{&nbsp;&nbsp;[3]=&gt;&nbsp;&nbsp;string(3)&nbsp;"def"&nbsp;}<br /></span><span style="color: #0000BB">$unset_arr</span><span style="color: #007700">[</span><span style="color: #0000BB">3</span><span style="color: #007700">]&nbsp;=&nbsp;</span><span style="color: #DD0000">"def"</span><span style="color: #007700">;&nbsp;</span><span style="color: #FF8000">//&nbsp;array()&nbsp;+&nbsp;array(3&nbsp;=&gt;&nbsp;"def")&nbsp;=&gt;&nbsp;array(3&nbsp;=&gt;&nbsp;"def")<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$unset_arr</span><span style="color: #007700">);<br /><br /></span><span style="color: #FF8000">//&nbsp;Object&nbsp;usage;&nbsp;creates&nbsp;new&nbsp;stdClass&nbsp;object&nbsp;(see&nbsp;http://www.php.net/manual/en/reserved.classes.php)<br />//&nbsp;Outputs:&nbsp;object(stdClass)#1&nbsp;(1)&nbsp;{&nbsp;&nbsp;["foo"]=&gt;&nbsp;&nbsp;string(3)&nbsp;"bar"&nbsp;}<br /></span><span style="color: #0000BB">$unset_obj</span><span style="color: #007700">-&gt;</span><span style="color: #0000BB">foo&nbsp;</span><span style="color: #007700">=&nbsp;</span><span style="color: #DD0000">'bar'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$unset_obj</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?&gt;</span>
</span>
</code></div>
     </div>

    </div><p>
   </p>
   <p class="para">
    Relying on the default value of an uninitialized variable is problematic
    in the case of including one file into another which uses the same
    variable name. It is also a major <a href="security.globals.php" class="link">security risk</a> with <a href="ini.core.php#ini.register-globals" class="link">register_globals</a> turned on. <a href="errorfunc.constants.php#errorfunc.constants.errorlevels.e-notice" class="link">E_NOTICE</a> level error is issued in case of
    working with uninitialized variables, however not in the case of appending
    elements to the uninitialized array. <a href="function.isset.php" class="function">isset()</a> language
    construct can be used to detect if a variable has been already initialized.
   </p>
  </div><?php manual_footer(); ?>
 
show source | credits | sitemap | contact | advertising | mirror sites