(PHP 4, PHP 5, PHP 7)
headers_sent — Checks if or where headers have been sent
&$file
[, int &$line
]] )Checks if or where headers have been sent.
You can't add any more header lines using the header() function once the header block has already been sent. Using this function you can at least prevent getting HTTP header related error messages. Another option is to use Output Buffering.
file
If the optional file and
line parameters are set,
headers_sent() will put the PHP source file name
and line number where output started in the file
and line variables.
lineThe line number where the output started.
headers_sent() will return FALSE if no HTTP headers
have already been sent or TRUE otherwise.
Example #1 Examples using headers_sent()
<?php
// If no headers are sent, send one
if (!headers_sent()) {
header('Location: http://www.example.com/');
exit;
}
// An example using the optional file and line parameters, as of PHP 4.3.0
// Note that $filename and $linenum are passed in for later use.
// Do not assign them values beforehand.
if (!headers_sent($filename, $linenum)) {
header('Location: http://www.example.com/');
exit;
// You would most likely trigger an error here.
} else {
echo "Headers already sent in $filename on line $linenum\n" .
"Cannot redirect, for now please click this <a " .
"href=\"http://www.example.com\">link</a> instead\n";
exit;
}
?>
Note:
数据头只会在SAPI支持时得到处理和输出。