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   debug extensions.
  </p> 

  <p class="para">
   The following table describes the contents of the major directories.
  </p>
  <table class="informaltable"> 
   <colgroup>

    <col align="left" width="1" />
    <col align="left" width="3" /> 
    <tbody valign="middle" class="tbody"> 
     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left">Directory</td> 
      <td colspan="1" rowspan="1" align="left">Contents</td> 
     </tr>
 
     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left"><var class="filename">php-src</var></td> 
      <td colspan="1" rowspan="1" align="left">
       Main PHP source files and main header files; here you&#039;ll find
       all of PHP&#039;s API definitions, macros, etc. (important). 
       Everything else is below this directory.
      </td> 
     </tr>
 
     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left"><var class="filename">php-src/ext</var></td> 
      <td colspan="1" rowspan="1" align="left">
       Repository for dynamic and built-in modules; by default, these
       are the &quot;official&quot; PHP modules that have been integrated into
       the main source tree. From PHP 4.0, it&#039;s possible to compile
       these standard extensions as dynamic loadable modules (at
       least, those that support it).
      </td> 
     </tr>
 
     <tr valign="middle">
      <td colspan="1" rowspan="1" align="left"><var class="filename">php-src/main</var></td>
      <td colspan="1" rowspan="1" align="left">
       This directory contains the main php macros and definitions. (important)
      </td>
     </tr>

     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left"><var class="filename">php-src/pear</var></td> 
      <td colspan="1" rowspan="1" align="left">
       Directory for the PHP Extension and Application Repository. This directory contains
       core PEAR files.
      </td> 
     </tr>
 
     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left"><var class="filename">php-src/sapi</var></td> 
      <td colspan="1" rowspan="1" align="left">
       Contains the code for the different server abstraction layers.
      </td> 
     </tr>
 
     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left"><var class="filename">TSRM</var></td>
      <td colspan="1" rowspan="1" align="left">
       Location of the &quot;Thread Safe Resource Manager&quot; (TSRM) for Zend
       and PHP.
      </td> 
     </tr>
 
     <tr valign="middle"> 
      <td colspan="1" rowspan="1" align="left"><var class="filename">ZendEngine2</var></td>
      <td colspan="1" rowspan="1" align="left">
       Location of the Zend Engine files; here you&#039;ll
       find all of Zend&#039;s API definitions, macros, etc. (important).
      </td> 
     </tr>
 
    </tbody> 
   </colgroup>
 
  </table> 
  <p class="para">
   Discussing all the files included in the PHP package is beyond the
   scope of this chapter. However, you should take a close look at the
   following files:<ul class="itemizedlist"> 
    <li class="listitem"> 
     <p class="para">
      <var class="filename">php-src/main/php.h</var>, located in the main PHP directory.
      This file contains most of PHP&#039;s macro and API definitions.
     </p> 
    </li> 
    <li class="listitem"> 
     <p class="para">
      <var class="filename">php-src/Zend/zend.h</var>, located in the main Zend directory.
      This file contains most of Zend&#039;s macros and definitions.
     </p> 
    </li> 
    <li class="listitem"> 
     <p class="para">
      <var class="filename">php-src/Zend/zend_API.h</var>, also located in the Zend
      directory, which defines Zend&#039;s API.
     </p> 
    </li> 
   </ul> You should also follow some sub-inclusions from
   these files; for example, the ones relating to the Zend executor,
   the PHP initialization file support, and such. After reading these
   files, take the time to navigate around the package a little to see
   the interdependencies of all files and modules - how they relate to
   each other and especially how they make use of each other. This
   also helps you to adapt to the coding style in which PHP is
   authored. To extend PHP, you should quickly adapt to this style.
  </p>
  
  <div id="internals2.ze1.zendapi.layout.conventions" class="sect3"> 
   <h4 class="title">Extension Conventions</h4> 
   <p class="para">
    Zend is built using certain conventions; to avoid breaking its
    standards, you should follow the rules described in the following
    sections.
   </p> 
  </div>

  <div id="internals2.ze1.zendapi.layout.macros" class="sect3"> 
   <h4 class="title">Macros</h4> 
   <p class="para">
    For almost every important task, Zend ships predefined macros that
    are extremely handy. The tables and figures in the following
    sections describe most of the basic functions, structures, and
    macros. The macro definitions can be found mainly in
    <var class="filename">zend.h</var> and <var class="filename">zend_API.h</var>.
    We suggest that you take a close look at these files after having
    studied this chapter. (Although you can go ahead and read them
    now, not everything will make sense to you yet.)
   </p>
  </div>
  
  <div id="internals2.ze1.zendapi.layout.memory-management" class="sect3"> 
   <h4 class="title">Memory Management</h4> 
   <p class="para">
    Resource management is a crucial issue, especially in server
    software. One of the most valuable resources is memory, and memory
    management should be handled with extreme care. Memory management
    has been partially abstracted in Zend, and you should stick to
    this abstraction for obvious reasons: Due to the abstraction, Zend
    gets full control over all memory allocations. Zend is able to
    determine whether a block is in use, automatically freeing unused
    blocks and blocks with lost references, and thus prevent memory
    leaks. The functions to be used are described in the following
    table: 
    <table class="informaltable"> 
     <colgroup>

      <col align="left" width="1" />
      <col align="left" width="1" /> 
      <tbody valign="middle" class="tbody"> 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left">Function</td> 
        <td colspan="1" rowspan="1" align="left">Description</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>emalloc()</b></td> 
        <td colspan="1" rowspan="1" align="left">Serves as replacement for
         <b>malloc()</b>.</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>efree()</b></td> 
        <td colspan="1" rowspan="1" align="left">Serves as replacement for
         <b>free()</b>.</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>estrdup()</b></td> 
        <td colspan="1" rowspan="1" align="left">Serves as replacement for
         <b>strdup()</b>.</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>estrndup()</b></td>
        <td colspan="1" rowspan="1" align="left">Serves as replacement for
         <b>strndup()</b>. Faster than
         <b>estrdup()</b> and binary-safe. This is the
         recommended function to use if you know the string length
         prior to duplicating it.</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>ecalloc()</b></td> 
        <td colspan="1" rowspan="1" align="left">Serves as replacement for
         <b>calloc()</b>.</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>erealloc()</b></td>
        <td colspan="1" rowspan="1" align="left">Serves as replacement for
         <b>realloc()</b>.</td> 
       </tr>
 
      </tbody> 
     </colgroup>
 
    </table> <b>emalloc()</b>,
    <b>estrdup()</b>,  <b>estrndup()</b>,
    <b>ecalloc()</b>, and <b>erealloc()</b>
    allocate internal memory; <b>efree()</b> frees these
    previously allocated blocks. Memory handled by the
    <b>e*()</b> functions is considered local to the
    current process and is discarded as soon as the script executed by
    this process is terminated. 
    <div class="warning"><b class="warning">Warning</b> 
     <p class="para">
      To allocate resident memory that survives termination of
      the current script, you can use <b>malloc()</b> and
      <b>free()</b>. This should only be done with extreme
      care, however, and only in conjunction with demands of the Zend
      API; otherwise, you risk memory leaks.
     </p> 
    </div>
    Zend also features a thread-safe resource manager to
    provide better native support for multithreaded Web servers. This
    requires you to allocate local structures for all of your global
    variables to allow concurrent threads to be run. Because the
    thread-safe mode of Zend was not finished back when this was written, 
    it is not yet extensively covered here.
   </p> 
  </div>

  <div id="internals2.ze1.zendapi.layout.dir-and-file" class="sect3"> 
   <h4 class="title">Directory and File Functions</h4> 
   <p class="para">
    The following directory and file functions should be used in Zend
    modules. They behave exactly like their C counterparts, but
    provide virtual working directory support on the thread level. 
    <table class="informaltable"> 
     <colgroup>

      <col align="left" width="0" />
      <col align="left" width="0" /> 
      <tbody valign="middle" class="tbody"> 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left">Zend Function</td> 
        <td colspan="1" rowspan="1" align="left">Regular C Function</td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_GETCWD()</b></td>
        <td colspan="1" rowspan="1" align="left"><a href="function.getcwd.html" class="function">getcwd()</a></td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_FOPEN()</b></td> 
        <td colspan="1" rowspan="1" align="left"><a href="function.fopen.html" class="function">fopen()</a></td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_OPEN()</b></td> 
        <td colspan="1" rowspan="1" align="left"><b>open()</b></td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_CHDIR()</b></td> 
        <td colspan="1" rowspan="1" align="left"><a href="function.chdir.html" class="function">chdir()</a></td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_GETWD()</b></td> 
        <td colspan="1" rowspan="1" align="left"><b>getwd()</b></td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_CHDIR_FILE()</b></td> 
        <td colspan="1" rowspan="1" align="left">
         Takes a file path as an argument and changes the current
         working directory to that file&#039;s directory.
        </td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_STAT()</b></td> 
        <td colspan="1" rowspan="1" align="left"><a href="function.stat.html" class="function">stat()</a></td> 
       </tr>
 
       <tr valign="middle"> 
        <td colspan="1" rowspan="1" align="left"><b>V_LSTAT()</b></td> 
        <td colspan="1" rowspan="1" align="left"><a href="function.lstat.html" class="function">lstat()</a></td> 
       </tr>
 
      </tbody> 
     </colgroup>
 
    </table></p> 
  </div>

  <div id="internals2.ze1.zendapi.layout.string-handling" class="sect3"> 
   <h4 class="title">String Handling</h4> 
   <p class="para">
    Strings are handled a bit differently by the Zend engine
    than other values such as integers, Booleans, etc., which don&#039;t require
    additional memory allocation for storing their values. If you want to
    return a string from a function, introduce a new string variable to the symbol
    table, or do something similar, you have to make sure that the memory the
    string will be occupying has previously been allocated, using the
    aforementioned <b>e*()</b> functions for allocation. (This might
    not make much sense to you yet; just keep it somewhere in your head for now - we&#039;ll get
    back to it shortly.)
   </p> 
  </div>

  <div id="internals2.ze1.zendapi.layout.complex-types" class="sect3"> 
   <h4 class="title">Complex Types</h4> 
   <p class="para">
    Complex types such as arrays and objects require
    different treatment. Zend features a single API for these types - they&#039;re
    stored using hash tables.
   </p> 
   <blockquote><p><b class="note">Note</b>: 
    
     To reduce complexity in the following source examples, we&#039;re only