function secure_input($val, $strip = 1, $xss = 1, $charset = 'UTF-8') 
{
  if (is_array($val))
  {
    $output = array();
    foreach ($val as $key => $data)
    {
      $output[$key] = secure_input($data, $strip, $charset);
    }
    return $output;
  }
  else
  {
    if ($xss > 0)
    {
      // code by nicolaspar
      $val = preg_replace('/([\x00-\x08][\x0b-\x0c][\x0e-\x20])/', '', $val);
      $search = 'abcdefghijklmnopqrstuvwxyz';
      $search .= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
      $search .= '1234567890!@#$%^&*()';
      $search .= '~`";:?+/={}[]-_|\'\\';
      
      for ($i = 0; $i < strlen($search); $i++)
      {
        $val = preg_replace('/(&#[x|X]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
        $val = preg_replace('/(&#0{0,8}'.ord($search[$i]).';?)/', $search[$i], $val); // with a ;
      }
      
      $ra1 = Array('javascript', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'blink', 'link', 'style', 'script', 'embed', 'object', 'iframe', 'frame', 'frameset', 'ilayer', 'layer', 'bgsound', 'title', 'base');
      $ra2 = Array('onabort', 'onactivate', 'onafterprint', 'onafterupdate', 'onbeforeactivate', 'onbeforecopy', 'onbeforecut', 'onbeforedeactivate', 'onbeforeeditfocus', 'onbeforepaste', 'onbeforeprint', 'onbeforeunload', 'onbeforeupdate', 'onblur', 'onbounce', 'oncellchange', 'onchange', 'onclick', 'oncontextmenu', 'oncontrolselect', 'oncopy', 'oncut', 'ondataavailable', 'ondatasetchanged', 'ondatasetcomplete', 'ondblclick', 'ondeactivate', 'ondrag', 'ondragend', 'ondragenter', 'ondragleave', 'ondragover', 'ondragstart', 'ondrop', 'onerror', 'onerrorupdate', 'onfilterchange', 'onfinish', 'onfocus', 'onfocusin', 'onfocusout', 'onhelp', 'onkeydown', 'onkeypress', 'onkeyup', 'onlayoutcomplete', 'onload', 'onlosecapture', 'onmousedown', 'onmouseenter', 'onmouseleave', 'onmousemove', 'onmouseout', 'onmouseover', 'onmouseup', 'onmousewheel', 'onmove', 'onmoveend', 'onmovestart', 'onpaste', 'onpropertychange', 'onreadystatechange', 'onreset', 'onresize', 'onresizeend', 'onresizestart', 'onrowenter', 'onrowexit', 'onrowsdelete', 'onrowsinserted', 'onscroll', 'onselect', 'onselectionchange', 'onselectstart', 'onstart', 'onstop', 'onsubmit', 'onunload');
      
      $ra = array_merge($ra1, $ra2);
      $found = true; 
      
      while ($found == true) 
      {
        $val_before = $val;
        for ($i = 0; $i < sizeof($ra); $i++) 
        {
          $pattern = '/';
          for ($j = 0; $j < strlen($ra[$i]); $j++)
          {
            if ($j > 0)
            {
              $pattern .= '(';
              $pattern .= '(&#[x|X]0{0,8}([9][a][b]);?)?';
              $pattern .= '|(&#0{0,8}([9][10][13]);?)?';
              $pattern .= ')?';
            }
            $pattern .= $ra[$i][$j];
          }
          $pattern .= '/i';
          $replacement = substr($ra[$i], 0, 2).'<x>'.substr($ra[$i], 2);
          $val = preg_replace($pattern, $replacement, $val);
          if ($val_before == $val) 
          {
            $found = false;
          }
        }
      }
    }
    
    // Strip HTML tags
    if ($strip > 0)
    {
      $val = strip_tags($val);
    }
    
    // Encode special chars
    $val = htmlentities($val, ENT_QUOTES, $charset);
    
    if(get_magic_quotes_gpc()) 
    {
        return mysql_real_escape_string(stripslashes($val));
    } 
    else
    {
        return mysql_real_escape_string($val);
    }
  }
} 
echo secure_input($foo);

function secure_input($val, $strip = 1, $charset = 'UTF-8') 
{
  if (is_array($val))
  {
    $output = array();
    foreach ($val as $key => $data)
    {
      $output[$key] = secure_input($data, $strip, $charset);
    }
    return $output;
  }
  else
  {
    // code by nicolaspar
    $val = preg_replace('/([\x00-\x08][\x0b-\x0c][\x0e-\x20])/', '', $val);
    $search = 'abcdefghijklmnopqrstuvwxyz';
    $search .= 'ABCDEFGHIJKLMNOPQRSTUVWXYZ';
    $search .= '1234567890!@#$%^&*()';
    $search .= '~`";:?+/={}[]-_|\'\\';
    
    for ($i = 0; $i < strlen($search); $i++)
    {
      $val = preg_replace('/(&#[x|X]0{0,8}'.dechex(ord($search[$i])).';?)/i', $search[$i], $val); // with a ;
      $val = preg_replace('/(&#0{0,8}'.ord($search[$i]).';?)/', $search[$i], $val); // with a ;
    }
    
    $ra1 = Array('javascript', 'vbscript', 'expression', 'applet', 'meta', 'xml', 'blink', 'link', 'style', 'script', 'embed', 'object', 'iframe', 'frame', 'frameset', 'ilayer', 'layer', 'bgsound', 'title', 'base');
    $ra2 = Array('onabort', 'onactivate', 'onafterprint', 'onafterupdate', 'onbeforeactivate', 'onbeforecopy', 'onbeforecut', 'onbeforedeactivate', 'onbeforeeditfocus', 'onbeforepaste', 'onbeforeprint', 'onbeforeunload', 'onbeforeupdate', 'onblur', 'onbounce', 'oncellchange', 'onchange', 'onclick', 'oncontextmenu', 'oncontrolselect', 'oncopy', 'oncut', 'ondataavailable', 'ondatasetchanged', 'ondatasetcomplete', 'ondblclick', 'ondeactivate', 'ondrag', 'ondragend', 'ondragenter', 'ondragleave', 'ondragover', 'ondragstart', 'ondrop', 'onerror', 'onerrorupdate', 'onfilterchange', 'onfinish', 'onfocus', 'onfocusin', 'onfocusout', 'onhelp', 'onkeydown', 'onkeypress', 'onkeyup', 'onlayoutcomplete', 'onload', 'onlosecapture', 'onmousedown', 'onmouseenter', 'onmouseleave', 'onmousemove', 'onmouseout', 'onmouseover', 'onmouseup', 'onmousewheel', 'onmove', 'onmoveend', 'onmovestart', 'onpaste', 'onpropertychange', 'onreadystatechange', 'onreset', 'onresize', 'onresizeend', 'onresizestart', 'onrowenter', 'onrowexit', 'onrowsdelete', 'onrowsinserted', 'onscroll', 'onselect', 'onselectionchange', 'onselectstart', 'onstart', 'onstop', 'onsubmit', 'onunload');
    
    $ra = array_merge($ra1, $ra2);
    $found = true; 
    
    while ($found == true) 
    {
      $val_before = $val;
      for ($i = 0; $i < sizeof($ra); $i++) 
      {
        $pattern = '/';
        for ($j = 0; $j < strlen($ra[$i]); $j++)
        {
          if ($j > 0)
          {
            $pattern .= '(';
            $pattern .= '(&#[x|X]0{0,8}([9][a][b]);?)?';
            $pattern .= '|(&#0{0,8}([9][10][13]);?)?';
            $pattern .= ')?';
          }
          $pattern .= $ra[$i][$j];
        }
        $pattern .= '/i';
        $replacement = substr($ra[$i], 0, 2).'<x>'.substr($ra[$i], 2);
        $val = preg_replace($pattern, $replacement, $val);
        if ($val_before == $val) 
        {
          $found = false;
        }
      }
    }
    
    // Strip HTML tags
    if ($strip > 0)
    {
      $val = strip_tags($val);
    }
    
    // Encode special chars
    $val = htmlentities($val, ENT_QUOTES, $charset);
    
    if(get_magic_quotes_gpc()) 
    {
        return mysql_real_escape_string(stripslashes($val));
    } 
    else
    {
        return mysql_real_escape_string($val);
    }
  }
} 
echo secure_input("testeando javascript:alert('hola');");

secure input (xss, sql injection)

sql, php

PHP