Call to undefined function apache_request_headers()

You can use the following replacement function:

<?php
if( !function_exists('apache_request_headers') ) {
///
function apache_request_headers() {
  $arh = array();
  $rx_http = '/\AHTTP_/';
  foreach($_SERVER as $key => $val) {
    if( preg_match($rx_http, $key) ) {
      $arh_key = preg_replace($rx_http, '', $key);
      $rx_matches = array();
      // do some nasty string manipulations to restore the original letter case
      // this should work in most cases
      $rx_matches = explode('_', $arh_key);
      if( count($rx_matches) > 0 and strlen($arh_key) > 2 ) {
        foreach($rx_matches as $ak_key => $ak_val) $rx_matches[$ak_key] = ucfirst($ak_val);
        $arh_key = implode('-', $rx_matches);
      }
      $arh[$arh_key] = $val;
    }
  }
  return( $arh );
}
///
}
///
?>

Source: PHP Manual

From the docs, before the release of PHP 5.4.0:

This function is only supported when PHP is installed as an Apache module.

PHP 5.4.0 and later support this function unconditionally.

Said docs also include replacement functions that mimic the functionality of apache_request_headers by stepping through $_SERVER.