How can this autoloader be improved?
Comments on your code


The access modifier for your constructor is missing.

function __construct...
public function __construct


Although no specified access modifier on methods means that PHP chooses public automatically (see PHP docs), I would add it for consistency.
Your autoloader does not cope with namespaces. You might want to read PSR-0, a specification stating how autoloaders should cope with namespaces, folders and filenames.
Do not add random newlines.

public function load($class)
{

    // expload the namespaces ex: foo\bar\tar array(foo, bar, tar)

public function load($class)
{
    // expload the namespaces ex: foo\bar\tar array(foo, bar, tar)

Do not add random spaces in comments.

// expload the namespaces ex: foo\bar\tar array(foo, bar, tar)
//get the last exploaded string and append .php so it becomes tar.php


Be consistent! I suggest using a space after //.
Be specific in variable names. $pathToClass lets me think that the string points to a specific class file. It does however point to a directory containing class files. I suggest using $classDir. $class is also an edge case since it does contain more than a single class name.
You don't need explode if you only wish to extract the class name.
Use strrpos():

$classFilename = substr($class, strrpos($class, '\\') + 1) . '.php';

Don't apply strtolower two times. Just save the result in a temporary variable. It makes your code DRY.
Since you declare pathToClass private anyway, consider marking the class final. Some arguments are provided in an article Final should be Default for Classes in Java (the same principles not only apply to Java, but also to many other languages including PHP).
Mark your load method private. It's an implementation detail, which shouldn't be accessible from outside the class.
Your load method is not required to return a boolean value. Returning true or false is not bad per se, maybe you will want to encapsulate the autoloading mechanism further and throw exceptions depending on the return value.




Elaborating on the PSR-0 specification

The spec defines mandatory requirements your autoloader must fulfill.

They provide example inputs your autoloader must accept:

\Doctrine\Common\IsolatedClassLoader =&gt; /path/to/project/lib/vendor/Doctrine/Common/IsolatedClassLoader.php
\Symfony\Core\Request =&gt; /path/to/project/lib/vendor/Symfony/Core/Request.php
\Zend\Acl =&gt; /path/to/project/lib/vendor/Zend/Acl.php
\Zend\Mail\Message =&gt; /path/to/project/lib/vendor/Zend/Mail/Message.php

\namespace\package\Class_Name =&gt; /path/to/project/lib/vendor/namespace/package/Class/Name.php
\namespace\package_name\Class_Name =&gt; /path/to/project/lib/vendor/namespace/package_name/Class/Name.php


Roughly speaking, your code is missing the following aspects:


Taking the namespace into account and using it to build a directory hierarchy.
Transforming underscores in class names to a directory hierarchy as well.