PHP檢測類是否存在方法是什麼？

PHP檢測類是否存在方法是什麼？

在PHP中可以使用「class_exists()」函數檢測類是否存，該函數的作用是檢查類是否已定義，其用法為「class_exists($class_name)」，其引數「$class_name」代表要檢測的類名。

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範例程式碼

<?php
// 使用前檢查類是否存在
if (class_exists('MyClass')) {
    $myclass = new MyClass();
}

?>

<?php
/**
* Set my include path here
*/
$include_path = array( '/include/this/dir', '/include/this/one/too' );
set_include_path( $include_path );
spl_autoload_register();
/**
* Assuming I have my own custom exception handler (MyException) let's
* try to see if a file exists.
*/
try {
    if( ! file_exists( 'myfile.php' ) ) {
        throw new MyException('Doh!');
    }
    include( 'myfile.php' );
}
catch( MyException $e ) {
    echo $e->getMessage();
}
/**
* The above code either includes myfile.php or throws the new MyException
* as expected. No problem right? The same should be true of class_exists(),
* right? So then...
*/
$classname = 'NonExistentClass';
try {
    if( ! class_exists( $classname ) ) {
        throw new MyException('Double Doh!');
    }
    $var = new $classname();
}
catch( MyException $e ) {
    echo $e->getMessage();
}
/**
* Should throw a new instance of MyException. But instead I get an
* uncaught LogicException blah blah blah for the default Exception
* class AND MyException. I only catch MyException so we've got on
* uncaught resulting in the dreaded LogicException error.
*/
?>

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