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(PHP >= 5.3.0, PECL phar >= 1.0.0)
Phar::offsetExists — determines whether a file exists in the phar
$offset 
   )This is an implementation of the ArrayAccess interface allowing direct manipulation of the contents of a Phar archive using array access brackets.
offsetExists() is called whenever isset() is called.
offset The filename (relative path) to look for in a Phar.
   Returns  TRUE  if the file exists within the phar, or  FALSE  if not.
  
Example #1 A Phar::offsetExists() example
  <?php
$p  = new  Phar ( dirname ( __FILE__ ) .  '/my.phar' ,  0 ,  'my.phar' );
 $p [ 'firstfile.txt' ] =  'first file' ;
 $p [ 'secondfile.txt' ] =  'second file' ;
 // the next set of lines call offsetExists() indirectly
 var_dump (isset( $p [ 'firstfile.txt' ]));
 var_dump (isset( $p [ 'nothere.txt' ]));
 ?>   以上例程会输出:
bool(true) bool(false)