diff --git a/system/libraries/Email.php b/system/libraries/Email.php
index be89d65..5049578 100644
--- a/system/libraries/Email.php
+++ b/system/libraries/Email.php
@@ -1514,14 +1514,7 @@
 		// which only works with "\n".
 		if ($this->crlf === "\r\n")
 		{
-			if (is_php('5.3'))
-			{
-				return quoted_printable_encode($str);
-			}
-			elseif (function_exists('imap_8bit'))
-			{
-				return imap_8bit($str);
-			}
+			return quoted_printable_encode($str);
 		}
 
 		// Reduce multiple spaces & remove nulls
diff --git a/system/libraries/Encryption.php b/system/libraries/Encryption.php
index a10a5c2..06284c2 100644
--- a/system/libraries/Encryption.php
+++ b/system/libraries/Encryption.php
@@ -152,10 +152,8 @@
 	public function __construct(array $params = array())
 	{
 		$this->_drivers = array(
-			'mcrypt' => defined('MCRYPT_DEV_URANDOM'),
-			// While OpenSSL is available for PHP 5.3.0, an IV parameter
-			// for the encrypt/decrypt functions is only available since 5.3.3
-			'openssl' => (is_php('5.3.3') && extension_loaded('openssl'))
+			'mcrypt'  => defined('MCRYPT_DEV_URANDOM'),
+			'openssl' => extension_loaded('openssl')
 		);
 
 		if ( ! $this->_drivers['mcrypt'] && ! $this->_drivers['openssl'])
diff --git a/system/libraries/Form_validation.php b/system/libraries/Form_validation.php
index 04445f5..25dc0d4 100644
--- a/system/libraries/Form_validation.php
+++ b/system/libraries/Form_validation.php
@@ -1216,18 +1216,7 @@
 			$str = 'ipv6.host'.substr($str, strlen($matches[1]) + 2);
 		}
 
-		$str = 'http://'.$str;
-
-		// There's a bug affecting PHP 5.2.13, 5.3.2 that considers the
-		// underscore to be a valid hostname character instead of a dash.
-		// Reference: https://bugs.php.net/bug.php?id=51192
-		if (version_compare(PHP_VERSION, '5.2.13', '==') OR version_compare(PHP_VERSION, '5.3.2', '=='))
-		{
-			sscanf($str, 'http://%[^/]', $host);
-			$str = substr_replace($str, strtr($host, array('_' => '-', '-' => '_')), 7, strlen($host));
-		}
-
-		return (filter_var($str, FILTER_VALIDATE_URL) !== FALSE);
+		return (filter_var('http://'.$str, FILTER_VALIDATE_URL) !== FALSE);
 	}
 
 	// --------------------------------------------------------------------
diff --git a/system/libraries/Migration.php b/system/libraries/Migration.php
index 316c94a..3e2107e 100644
--- a/system/libraries/Migration.php
+++ b/system/libraries/Migration.php
@@ -288,10 +288,7 @@
 				$this->_error_string = sprintf($this->lang->line('migration_class_doesnt_exist'), $class);
 				return FALSE;
 			}
-			// method_exists() returns true for non-public methods,
-			// while is_callable() can't be used without instantiating.
-			// Only get_class_methods() satisfies both conditions.
-			elseif ( ! in_array($method, array_map('strtolower', get_class_methods($class))))
+			elseif ( ! is_callable(array($class, $method)))
 			{
 				$this->_error_string = sprintf($this->lang->line('migration_missing_'.$method.'_method'), $class);
 				return FALSE;
diff --git a/system/libraries/Session/drivers/Session_files_driver.php b/system/libraries/Session/drivers/Session_files_driver.php
index 57c3777..bf4df8b 100644
--- a/system/libraries/Session/drivers/Session_files_driver.php
+++ b/system/libraries/Session/drivers/Session_files_driver.php
@@ -149,18 +149,9 @@
 		// which re-reads session data
 		if ($this->_file_handle === NULL)
 		{
-			// Just using fopen() with 'c+b' mode would be perfect, but it is only
-			// available since PHP 5.2.6 and we have to set permissions for new files,
-			// so we'd have to hack around this ...
-			if (($this->_file_new = ! file_exists($this->_file_path.$session_id)) === TRUE)
-			{
-				if (($this->_file_handle = fopen($this->_file_path.$session_id, 'w+b')) === FALSE)
-				{
-					log_message('error', "Session: File '".$this->_file_path.$session_id."' doesn't exist and cannot be created.");
-					return $this->_failure;
-				}
-			}
-			elseif (($this->_file_handle = fopen($this->_file_path.$session_id, 'r+b')) === FALSE)
+			$this->_file_new = ! file_exists($this->_file_path.$session_id);
+
+			if (($this->_file_handle = fopen($this->_file_path.$session_id, 'c+b')) === FALSE)
 			{
 				log_message('error', "Session: Unable to open file '".$this->_file_path.$session_id."'.");
 				return $this->_failure;
diff --git a/system/libraries/Upload.php b/system/libraries/Upload.php
index 056f6de..a8cf752 100644
--- a/system/libraries/Upload.php
+++ b/system/libraries/Upload.php
@@ -1086,13 +1086,7 @@
 		if (memory_get_usage() && ($memory_limit = ini_get('memory_limit')))
 		{
 			$memory_limit *= 1024 * 1024;
-
-			// There was a bug/behavioural change in PHP 5.2, where numbers over one million get output
-			// into scientific notation. number_format() ensures this number is an integer
-			// http://bugs.php.net/bug.php?id=43053
-
-			$memory_limit = number_format(ceil(filesize($file) + $memory_limit), 0, '.', '');
-
+			$memory_limit = (int) ceil(filesize($file) + $memory_limit);
 			ini_set('memory_limit', $memory_limit); // When an integer is used, the value is measured in bytes. - PHP.net
 		}
 
@@ -1207,28 +1201,21 @@
 		// We'll need this to validate the MIME info string (e.g. text/plain; charset=us-ascii)
 		$regexp = '/^([a-z\-]+\/[a-z0-9\-\.\+]+)(;\s.+)?$/';
 
-		/* Fileinfo extension - most reliable method
-		 *
-		 * Unfortunately, prior to PHP 5.3 - it's only available as a PECL extension and the
-		 * more convenient FILEINFO_MIME_TYPE flag doesn't exist.
-		 */
-		if (function_exists('finfo_file'))
+		// Fileinfo extension - most reliable method
+		$finfo = @finfo_open(FILEINFO_MIME);
+		if (is_resource($finfo)) // It is possible that a FALSE value is returned, if there is no magic MIME database file found on the system
 		{
-			$finfo = @finfo_open(FILEINFO_MIME);
-			if (is_resource($finfo)) // It is possible that a FALSE value is returned, if there is no magic MIME database file found on the system
-			{
-				$mime = @finfo_file($finfo, $file['tmp_name']);
-				finfo_close($finfo);
+			$mime = @finfo_file($finfo, $file['tmp_name']);
+			finfo_close($finfo);
 
-				/* According to the comments section of the PHP manual page,
-				 * it is possible that this function returns an empty string
-				 * for some files (e.g. if they don't exist in the magic MIME database)
-				 */
-				if (is_string($mime) && preg_match($regexp, $mime, $matches))
-				{
-					$this->file_type = $matches[1];
-					return;
-				}
+			/* According to the comments section of the PHP manual page,
+			 * it is possible that this function returns an empty string
+			 * for some files (e.g. if they don't exist in the magic MIME database)
+			 */
+			if (is_string($mime) && preg_match($regexp, $mime, $matches))
+			{
+				$this->file_type = $matches[1];
+				return;
 			}
 		}