diff --git a/includes/parser/AbuseFilterCachingParser.php b/includes/parser/AbuseFilterCachingParser.php
index e76a9b406..6b9ee40dd 100644
--- a/includes/parser/AbuseFilterCachingParser.php
+++ b/includes/parser/AbuseFilterCachingParser.php
@@ -356,6 +356,16 @@ class AbuseFilterCachingParser extends AbuseFilterParser {
 				throw new AFPUserVisibleException( 'unrecognisedvar', $node->position, [ $varName ] );
 			}
 			$this->setUserVariable( $varName, new AFPData( AFPData::DUNDEFINED ) );
+		} elseif (
+			$node->type === AFPTreeNode::FUNCTION_CALL &&
+			in_array( $node->children[0], [ 'set', 'set_var' ] ) &&
+			isset( $node->children[1] )
+		) {
+			$varnameNode = $node->children[1];
+			if ( $varnameNode->type === AFPTreeNode::ATOM ) {
+				// @todo Handle expressions here
+				$this->setUserVariable( $varnameNode->children->value, new AFPData( AFPData::DUNDEFINED ) );
+			}
 		} elseif ( $node->type === AFPTreeNode::ATOM ) {
 			return;
 		}
diff --git a/includes/parser/AbuseFilterParser.php b/includes/parser/AbuseFilterParser.php
index e732f7c24..b4a21efb3 100644
--- a/includes/parser/AbuseFilterParser.php
+++ b/includes/parser/AbuseFilterParser.php
@@ -243,18 +243,34 @@ class AbuseFilterParser {
 				}
 			} elseif ( $this->mCur->type === AFPToken::TID ) {
 				// T214674, define variables
-				$varname = $this->mCur->value;
 				$next = $this->getNextToken();
-				if ( $next->type === AFPToken::TOP && $next->value === ':=' ) {
-					$this->setUserVariable( $varname, new AFPData( AFPData::DUNDEFINED ) );
-				} elseif ( $next->type === AFPToken::TSQUAREBRACKET && $next->value === '[' ) {
-					if ( !$this->mVariables->varIsSet( $varname ) ) {
-						throw new AFPUserVisibleException( 'unrecognisedvar',
-							$next->pos,
-							[ $varname ]
-						);
+				if (
+					in_array( $this->mCur->value, [ 'set', 'set_var' ] ) &&
+					$next->type === AFPToken::TBRACE && $next->value === '('
+				) {
+					// This is for setter functions.
+					$this->move();
+					$braces++;
+					$next = $this->getNextToken();
+					if ( $next->type === AFPToken::TSTRING ) {
+						// @todo Handle expressions here
+						$this->setUserVariable( $next->value, new AFPData( AFPData::DUNDEFINED ) );
+					}
+				} else {
+					// Simple assignment with :=
+					$varname = $this->mCur->value;
+					$next = $this->getNextToken();
+					if ( $next->type === AFPToken::TOP && $next->value === ':=' ) {
+						$this->setUserVariable( $varname, new AFPData( AFPData::DUNDEFINED ) );
+					} elseif ( $next->type === AFPToken::TSQUAREBRACKET && $next->value === '[' ) {
+						if ( !$this->mVariables->varIsSet( $varname ) ) {
+							throw new AFPUserVisibleException( 'unrecognisedvar',
+								$next->pos,
+								[ $varname ]
+							);
+						}
+						$this->setUserVariable( $varname, new AFPData( AFPData::DUNDEFINED ) );
 					}
-					$this->setUserVariable( $varname, new AFPData( AFPData::DUNDEFINED ) );
 				}
 			}
 		}
diff --git a/tests/phpunit/AbuseFilterParserTest.php b/tests/phpunit/AbuseFilterParserTest.php
index 03b727c5c..2b077286e 100644
--- a/tests/phpunit/AbuseFilterParserTest.php
+++ b/tests/phpunit/AbuseFilterParserTest.php
@@ -226,6 +226,8 @@ class AbuseFilterParserTest extends AbuseFilterParserTestCase {
 			[ 'a := [1,2', 'doLevelAtom' ],
 			[ '1 = 1 | (', 'skipOverBraces' ],
 			[ 'a := [1,2,3]; 3 = a[5', 'doLevelArrayElements' ],
+			// `set` is the name of a function. It's unclear whether the following should be allowed.
+			[ 'set:= 1; set contains 1', 'doLevelFunction' ],
 		];
 	}
 
@@ -739,12 +741,15 @@ class AbuseFilterParserTest extends AbuseFilterParserTestCase {
 			[ "x := [5]; false & (1 == 1; y := 'b'; x[1] := 'x'; 3 < 4); y != 'b' & x[1] != 'x'" ],
 			[ "(var := [1]); false & ( var[] := 'baz' ); var contains 'baz'" ],
 			[ "(var := [1]); false & ( var[1] := 'baz' ); var[1] === 'baz'" ],
+			[ "false & (set('myvar', 1)); myvar contains 1" ],
 			// The following tests are to ensure that we don't get a match
 			[ "false & ( var := 'foo'; x := get_matches( var, added_lines )[1] ); x != false" ],
 			[ "false & ( var := 'foo'); var !== null" ],
 			[ "false & ( var := 'foo'); var === null" ],
+			[ "false & (set('myvar', 'foo')); myvar === 'foo' | myvar !== 'foo'" ],
 			[ "false & ( var := 'foo'); var[0] !== 123456" ],
 			[ "false & ( var := 'foo'); var[0][123] !== 123456" ],
+			[ "false & (set('myvar', 'foo')); myvar[1][2] === 'foo' | myvar[1][2] !== 'foo'" ],
 			// Identifier before closing skipped brace, T214674#5374757
 			[ "false & ( var := 'foo'; 'x' in var )" ],
 			[ "false & ( var := 'foo'; added_lines irlike var )" ],