diff --git a/extensions/php/src/features/completionItemProvider.ts b/extensions/php/src/features/completionItemProvider.ts
index a430aa3f154..3d918dc4cf5 100644
--- a/extensions/php/src/features/completionItemProvider.ts
+++ b/extensions/php/src/features/completionItemProvider.ts
@@ -27,7 +27,7 @@ export default class PHPCompletionItemProvider implements CompletionItemProvider
 		}
 
 		var added: any = {};
-		var createNewProposal = function (kind: CompletionItemKind, name: string, entry: phpGlobals.IEntry): CompletionItem {
+		var createNewProposal = function (kind: CompletionItemKind, name: string, entry: phpGlobals.IEntry | null): CompletionItem {
 			var proposal: CompletionItem = new CompletionItem(name);
 			proposal.kind = kind;
 			if (entry) {
@@ -85,7 +85,7 @@ export default class PHPCompletionItemProvider implements CompletionItemProvider
 		var text = document.getText();
 		if (prefix[0] === '$') {
 			var variableMatch = /\$([a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)/g;
-			var match: RegExpExecArray = null;
+			var match: RegExpExecArray | null = null;
 			while (match = variableMatch.exec(text)) {
 				var word = match[0];
 				if (!added[word]) {
@@ -95,7 +95,7 @@ export default class PHPCompletionItemProvider implements CompletionItemProvider
 			}
 		}
 		var functionMatch = /function\s+([a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*)\s*\(/g;
-		var match: RegExpExecArray = null;
+		var match: RegExpExecArray | null = null;
 		while (match = functionMatch.exec(text)) {
 			var word = match[1];
 			if (!added[word]) {
diff --git a/extensions/php/src/features/hoverProvider.ts b/extensions/php/src/features/hoverProvider.ts
index 1e300e2fccd..2e0a137ec0a 100644
--- a/extensions/php/src/features/hoverProvider.ts
+++ b/extensions/php/src/features/hoverProvider.ts
@@ -11,10 +11,10 @@ import { textToMarkedString } from './utils/markedTextUtil';
 
 export default class PHPHoverProvider implements HoverProvider {
 
-	public provideHover(document: TextDocument, position: Position, token: CancellationToken): Hover {
+	public provideHover(document: TextDocument, position: Position, token: CancellationToken): Hover | undefined {
 		let enable = workspace.getConfiguration('php').get<boolean>('suggest.basic', true);
 		if (!enable) {
-			return null;
+			return;
 		}
 
 		let wordRange = document.getWordRangeAtPosition(position);
diff --git a/extensions/php/src/features/signatureHelpProvider.ts b/extensions/php/src/features/signatureHelpProvider.ts
index b52392cfca4..42c14b8066a 100644
--- a/extensions/php/src/features/signatureHelpProvider.ts
+++ b/extensions/php/src/features/signatureHelpProvider.ts
@@ -69,7 +69,7 @@ class BackwardIterator {
 
 export default class PHPSignatureHelpProvider implements SignatureHelpProvider {
 
-	public provideSignatureHelp(document: TextDocument, position: Position, token: CancellationToken): Promise<SignatureHelp> {
+	public provideSignatureHelp(document: TextDocument, position: Position, token: CancellationToken): Promise<SignatureHelp> | null {
 		let enable = workspace.getConfiguration('php').get<boolean>('suggest.basic', true);
 		if (!enable) {
 			return null;
@@ -95,7 +95,7 @@ export default class PHPSignatureHelpProvider implements SignatureHelpProvider {
 		let signatureInfo = new SignatureInformation(ident + paramsString, entry.description);
 
 		var re = /\w*\s+\&?\$[\w_\.]+|void/g;
-		var match: RegExpExecArray = null;
+		var match: RegExpExecArray | null = null;
 		while ((match = re.exec(paramsString)) !== null) {
 			signatureInfo.parameters.push({ label: match[0], documentation: '' });
 		}
diff --git a/extensions/php/src/features/utils/async.ts b/extensions/php/src/features/utils/async.ts
index f9c86930453..5093f7c3eba 100644
--- a/extensions/php/src/features/utils/async.ts
+++ b/extensions/php/src/features/utils/async.ts
@@ -29,9 +29,9 @@ export interface ITask<T> {
  */
 export class Throttler<T> {
 
-	private activePromise: Promise<T>;
-	private queuedPromise: Promise<T>;
-	private queuedPromiseFactory: ITask<Promise<T>>;
+	private activePromise: Promise<T> | null;
+	private queuedPromise: Promise<T> | null;
+	private queuedPromiseFactory: ITask<Promise<T>> | null;
 
 	constructor() {
 		this.activePromise = null;
@@ -47,26 +47,26 @@ export class Throttler<T> {
 				var onComplete = () => {
 					this.queuedPromise = null;
 
-					var result = this.queue(this.queuedPromiseFactory);
+					var result = this.queue(this.queuedPromiseFactory!);
 					this.queuedPromiseFactory = null;
 
 					return result;
 				};
 
 				this.queuedPromise = new Promise<T>((resolve, reject) => {
-					this.activePromise.then(onComplete, onComplete).then(resolve);
+					this.activePromise!.then(onComplete, onComplete).then(resolve);
 				});
 			}
 
 			return new Promise<T>((resolve, reject) => {
-				this.queuedPromise.then(resolve, reject);
+				this.queuedPromise!.then(resolve, reject);
 			});
 		}
 
 		this.activePromise = promiseFactory();
 
 		return new Promise<T>((resolve, reject) => {
-			this.activePromise.then((result: T) => {
+			this.activePromise!.then((result: T) => {
 				this.activePromise = null;
 				resolve(result);
 			}, (err: any) => {
@@ -103,10 +103,10 @@ export class Throttler<T> {
 export class Delayer<T> {
 
 	public defaultDelay: number;
-	private timeout: NodeJS.Timer;
-	private completionPromise: Promise<T>;
-	private onResolve: (value: T | Thenable<T>) => void;
-	private task: ITask<T>;
+	private timeout: NodeJS.Timer | null;
+	private completionPromise: Promise<T> | null;
+	private onResolve: ((value: T | Thenable<T> | undefined) => void) | null;
+	private task: ITask<T> | null;
 
 	constructor(defaultDelay: number) {
 		this.defaultDelay = defaultDelay;
@@ -127,7 +127,7 @@ export class Delayer<T> {
 				this.completionPromise = null;
 				this.onResolve = null;
 
-				var result = this.task();
+				var result = this.task!();
 				this.task = null;
 
 				return result;
@@ -136,7 +136,7 @@ export class Delayer<T> {
 
 		this.timeout = setTimeout(() => {
 			this.timeout = null;
-			this.onResolve(null);
+			this.onResolve!(undefined);
 		}, delay);
 
 		return this.completionPromise;
diff --git a/extensions/php/src/features/validationProvider.ts b/extensions/php/src/features/validationProvider.ts
index 712164af0f9..0a26bbd5a9b 100644
--- a/extensions/php/src/features/validationProvider.ts
+++ b/extensions/php/src/features/validationProvider.ts
@@ -16,7 +16,7 @@ let localize = nls.loadMessageBundle();
 
 export class LineDecoder {
 	private stringDecoder: NodeStringDecoder;
-	private remaining: string;
+	private remaining: string | null;
 
 	constructor(encoding: string = 'utf8') {
 		this.stringDecoder = new StringDecoder(encoding);
@@ -55,7 +55,7 @@ export class LineDecoder {
 		return result;
 	}
 
-	public end(): string {
+	public end(): string | null {
 		return this.remaining;
 	}
 }
@@ -88,17 +88,17 @@ export default class PHPValidationProvider {
 	private static FileArgs: string[] = ['-l', '-n', '-d', 'display_errors=On', '-d', 'log_errors=Off', '-f'];
 
 	private validationEnabled: boolean;
-	private executableIsUserDefined: boolean;
-	private executable: string;
+	private executableIsUserDefined: boolean | undefined;
+	private executable: string | undefined;
 	private trigger: RunTrigger;
 	private pauseValidation: boolean;
 
-	private documentListener: vscode.Disposable;
+	private documentListener: vscode.Disposable | null;
 	private diagnosticCollection: vscode.DiagnosticCollection;
 	private delayers: { [key: string]: ThrottledDelayer<void> };
 
 	constructor(private workspaceStore: vscode.Memento) {
-		this.executable = null;
+		this.executable = undefined;
 		this.validationEnabled = true;
 		this.trigger = RunTrigger.onSave;
 		this.pauseValidation = false;
@@ -145,7 +145,7 @@ export default class PHPValidationProvider {
 			}
 			this.trigger = RunTrigger.from(section.get<string>('validate.run', RunTrigger.strings.onSave));
 		}
-		if (this.executableIsUserDefined !== true && this.workspaceStore.get<string>(CheckedExecutablePath, undefined) !== void 0) {
+		if (this.executableIsUserDefined !== true && this.workspaceStore.get<string | undefined>(CheckedExecutablePath, undefined) !== void 0) {
 			vscode.commands.executeCommand('setContext', 'php.untrustValidationExecutableContext', true);
 		}
 		this.delayers = Object.create(null);
@@ -195,7 +195,7 @@ export default class PHPValidationProvider {
 		};
 
 		if (this.executableIsUserDefined !== void 0 && !this.executableIsUserDefined) {
-			let checkedExecutablePath = this.workspaceStore.get<string>(CheckedExecutablePath, undefined);
+			let checkedExecutablePath = this.workspaceStore.get<string | undefined>(CheckedExecutablePath, undefined);
 			if (!checkedExecutablePath || checkedExecutablePath !== this.executable) {
 				vscode.window.showInformationMessage<MessageItem>(
 					localize('php.useExecutablePath', 'Do you allow {0} (defined as a workspace setting) to be executed to lint PHP files?', this.executable),
@@ -286,7 +286,7 @@ export default class PHPValidationProvider {
 	}
 
 	private showError(error: any, executable: string): void {
-		let message: string = null;
+		let message: string | null = null;
 		if (error.code === 'ENOENT') {
 			if (this.executable) {
 				message = localize('wrongExecutable', 'Cannot validate since {0} is not a valid php executable. Use the setting \'php.validate.executablePath\' to configure the PHP executable.', executable);
@@ -296,6 +296,8 @@ export default class PHPValidationProvider {
 		} else {
 			message = error.message ? error.message : localize('unknownReason', 'Failed to run php using path: {0}. Reason is unknown.', executable);
 		}
-		vscode.window.showInformationMessage(message);
+		if (message) {
+			vscode.window.showInformationMessage(message);
+		}
 	}
 }
diff --git a/extensions/php/tsconfig.json b/extensions/php/tsconfig.json
index a2b5bcdfddf..b16347a7524 100644
--- a/extensions/php/tsconfig.json
+++ b/extensions/php/tsconfig.json
@@ -5,7 +5,8 @@
 			"es2015"
 		],
 		"module": "commonjs",
-		"outDir": "./out"
+		"outDir": "./out",
+		"strict": true
 	},
 	"include": [
 		"src/**/*"