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Types of filters
PHP Manual

## Filter flags

List of filter flags
ID | Used with | Description
--- | --- | ---
`FILTER_FLAG_STRIP_LOW` | `FILTER_SANITIZE_ENCODED`, `FILTER_SANITIZE_SPECIAL_CHARS`, `FILTER_SANITIZE_STRING`, `FILTER_UNSAFE_RAW` | Strips characters that have a numerical value <32.
`FILTER_FLAG_STRIP_HIGH` | `FILTER_SANITIZE_ENCODED`, `FILTER_SANITIZE_SPECIAL_CHARS`, `FILTER_SANITIZE_STRING`, `FILTER_UNSAFE_RAW` | Strips characters that have a numerical value >127.
`FILTER_FLAG_ALLOW_FRACTION` | `FILTER_SANITIZE_NUMBER_FLOAT` | Allows a period (.) as a fractional separator in numbers.
`FILTER_FLAG_ALLOW_THOUSAND` | `FILTER_SANITIZE_NUMBER_FLOAT`, `FILTER_VALIDATE_FLOAT` | Allows a comma (,) as a thousands separator in numbers.
`FILTER_FLAG_ALLOW_SCIENTIFIC` | `FILTER_SANITIZE_NUMBER_FLOAT` | Allows an e or E for scientific notation in numbers.
`FILTER_FLAG_NO_ENCODE_QUOTES` | `FILTER_SANITIZE_STRING` | If this flag is present, single (') and double (") quotes will not be encoded.
`FILTER_FLAG_ENCODE_LOW` | `FILTER_SANITIZE_ENCODED`, `FILTER_SANITIZE_STRING`, `FILTER_SANITIZE_RAW` | Encodes all characters with a numerical value <32.
`FILTER_FLAG_ENCODE_HIGH` | `FILTER_SANITIZE_ENCODED`, `FILTER_SANITIZE_SPECIAL_CHARS`, `FILTER_SANITIZE_STRING`, `FILTER_SANITIZE_RAW` | Encodes all characters with a numerical value >127.
`FILTER_FLAG_ENCODE_AMP` | `FILTER_SANITIZE_STRING`, `FILTER_SANITIZE_RAW` | Encodes ampersands (&).
`FILTER_NULL_ON_FAILURE` | `FILTER_VALIDATE_BOOLEAN` | Returns `NULL` for unrecognized boolean values.
`FILTER_FLAG_ALLOW_OCTAL` | `FILTER_VALIDATE_INT` | Regards inputs starting with a zero (0) as octal numbers. This only allows the succeeding digits to be 0-7.
`FILTER_FLAG_ALLOW_HEX` | `FILTER_VALIDATE_INT` | Regards inputs starting with 0x or 0X as hexadecimal numbers. This only allows succeeding characters to be a-fA-F0-9.
`FILTER_FLAG_IPV4` | `FILTER_VALIDATE_IP` | Allows the IP address to be in IPv4 format.
`FILTER_FLAG_IPV6` | `FILTER_VALIDATE_IP` | Allows the IP address to be in IPv6 format.
`FILTER_FLAG_NO_PRIV_RANGE` | `FILTER_VALIDATE_IP` | Fails validation for the following private IPv4 ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Fails validation for the IPv6 addresses starting with FD or FC.
`FILTER_FLAG_NO_RES_RANGE` | `FILTER_VALIDATE_IP` | Fails validation for the following reserved IPv4 ranges: 0.0.0.0/8, 169.254.0.0/16, 192.0.2.0/24 and 224.0.0.0/4. This flag does not apply to IPv6 addresses.
`FILTER_FLAG_PATH_REQUIRED` | `FILTER_VALIDATE_URL` | Requires the URL to contain a path part.
`FILTER_FLAG_QUERY_REQUIRED` | `FILTER_VALIDATE_URL` | Requires the URL to contain a query string.

Other filters
Predefined Constants
Types of filters
PHP Manual