I get similar error messages when attempting a fresh install of SS 2.3.7 and 2.4 in Snow Leopard (Cannot modify header information). Managed many installs (with Leopard) before. De-zipped packages with "The Uninstaller". Ran permissions.

This error is when installing 2.4. Previous SS installs work fine locally.

* Director::set_environment_type(dev/test/development)
Line 56 of ConfigureFromEnv.php
* require_once(/Users/xxx/Sites/silverstripetest/sapphire/conf/ConfigureFromEnv.php)
Line 9 of _config.php
* require_once(/Users/xxx/Sites/silverstripetest/mysite/_config.php)
Line 5846 of manifest-main
* require_once(/private/var/folders/Y8/Y81DLnCIHvKzt-TFDyycZE+++TI/-Tmp-/silverstripe-cache-Users-a-Sites-silverstripetest/manifest-main)
Line 70 of ManifestBuilder.php
* ManifestBuilder::include_manifest()
Line 150 of Core.php
* require_once(/Users/xxx/Sites/silverstripetest/sapphire/core/Core.php)
Line 61 of main.php

[Warning] Cannot modify header information - headers already sent by (output started at /Users/xxx/Sites/silverstripetest/sapphire/dev/Debug.php:515)
GET /silverstripetest/

Line 107 in /Users/xxx/Sites/silverstripetest/sapphire/main.php
Source

98 }
99
100 // Connect to database
101 require_once("core/model/DB.php");
102
103 // Redirect to the installer if no database is selected
104 if(!isset($databaseConfig) || !isset($databaseConfig['database']) || !$databaseConfig['database']) {
105    $s = (isset($_SERVER['SSL']) || (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] != 'off')) ? 's' : '';
106    $installURL = "http$s://" . $_SERVER['HTTP_HOST'] . dirname(dirname($_SERVER['SCRIPT_NAME'])) . '/install.php';
107    header("Location: $installURL");
108    die();
109 }
110
111 if (isset($_GET['debug_profile'])) Profiler::mark('DB::connect');
112 DB::connect($databaseConfig);
113 if (isset($_GET['debug_profile'])) Profiler::unmark('DB::connect');

Trace

* header(Location: http://localhost/silverstripetest/install.php)
Line 107 of main.php

It could be that you have a php file with spaces after the closing ?>

It's recommended to removed the closing ?> altogether