diff --git a/.phpcs.xml b/.phpcs.xml
index 58486753..0c36fa5b 100644
--- a/.phpcs.xml
+++ b/.phpcs.xml
@@ -40,6 +40,7 @@
     <rule ref="SlevomatCodingStandard.Commenting.EmptyComment" />
     <rule ref="SlevomatCodingStandard.Commenting.UselessFunctionDocComment" />
     <rule ref="Generic.Formatting.SpaceAfterCast" />
+    <rule ref="Squiz.Strings.DoubleQuoteUsage" />
     <rule ref="SlevomatCodingStandard.TypeHints.DeclareStrictTypes">
         <exclude-pattern>/includes</exclude-pattern>
         <properties>
diff --git a/includes/pages/admin_user.php b/includes/pages/admin_user.php
index d562677d..7f2f6874 100644
--- a/includes/pages/admin_user.php
+++ b/includes/pages/admin_user.php
@@ -65,7 +65,7 @@ function admin_user()
             $html .= '  <tr><td>' . __('DECT') . '</td><td>' . '<input size="40" name="eDECT" value="' . $user_source->contact->dect . '" class="form-control" maxlength="40"></td></tr>' . "\n";
         }
         if ($user_source->settings->email_human) {
-            $html .= "  <tr><td>" . __('settings.profile.email') . "</td><td>" . '<input type="email" size="40" name="eemail" value="' . $user_source->email . '" class="form-control" maxlength="254"></td></tr>' . "\n";
+            $html .= '  <tr><td>' . __('settings.profile.email') . '</td><td>' . '<input type="email" size="40" name="eemail" value="' . $user_source->email . '" class="form-control" maxlength="254"></td></tr>' . "\n";
         }
         if (config('enable_tshirt_size') && !config('other_goodie')) {
             $html .= '  <tr><td>' . __('user.shirt_size') . '</td><td>'
@@ -178,7 +178,7 @@ function admin_user()
             button(user_delete_link($user_source->id), icon('trash') . __('delete'), 'btn-danger')
         ]);
 
-        $html .= "<hr />";
+        $html .= '<hr>';
     } else {
         switch ($request->input('action')) {
             case 'save_groups':
diff --git a/includes/sys_page.php b/includes/sys_page.php
index 9909b697..a5eb7142 100644
--- a/includes/sys_page.php
+++ b/includes/sys_page.php
@@ -249,7 +249,7 @@ function strip_request_item_nl($name, $default_value = null)
 function strip_item($item)
 {
     // Only allow letters, symbols, punctuation, separators and numbers without html tags
-    return preg_replace("/([^\p{L}\p{S}\p{P}\p{Z}\p{N}+]+)/ui", '', strip_tags($item));
+    return preg_replace('/([^\p{L}\p{S}\p{P}\p{Z}\p{N}+]+)/ui', '', strip_tags($item));
 }
 
 /**
diff --git a/includes/sys_template.php b/includes/sys_template.php
index 49915baf..db789d87 100644
--- a/includes/sys_template.php
+++ b/includes/sys_template.php
@@ -340,7 +340,7 @@ function render_table($columns, $rows, $data = true)
     foreach ($rows as $row) {
         $html .= '<tr>';
         foreach ($columns as $key => $column) {
-            $value = "&nbsp;";
+            $value = '&nbsp;';
             if (isset($row[$key])) {
                 $value = $row[$key];
             }
diff --git a/includes/view/ShiftCalendarShiftRenderer.php b/includes/view/ShiftCalendarShiftRenderer.php
index e1f104bb..72f56f00 100644
--- a/includes/view/ShiftCalendarShiftRenderer.php
+++ b/includes/view/ShiftCalendarShiftRenderer.php
@@ -197,11 +197,11 @@ class ShiftCalendarShiftRenderer
                 ? $inner_text . icon('mortarboard-fill')
                 // Add link to join the angeltype first
                 : $inner_text . '<br />'
-                    . button(
-                        page_link_to('user_angeltypes', ['action' => 'add', 'angeltype_id' => $angeltype->id]),
-                        sprintf(__('Become %s'), $angeltype->name),
-                        'btn-sm'
-                    ),
+                . button(
+                    page_link_to('user_angeltypes', ['action' => 'add', 'angeltype_id' => $angeltype->id]),
+                    sprintf(__('Become %s'), $angeltype->name),
+                    'btn-sm'
+                ),
             // Shift collides or user is already signed up: No signup allowed
             ShiftSignupStatus::COLLIDES, ShiftSignupStatus::SIGNED_UP => $inner_text,
             // Shift is full
@@ -251,12 +251,12 @@ class ShiftCalendarShiftRenderer
                     button(
                         page_link_to('user_shifts', ['edit_shift' => $shift->id]),
                         icon('pencil'),
-                        "btn-$class btn-sm border-light text-white"
+                        'btn-' . $class . ' btn-sm border-light text-white'
                     ),
                     button(
                         page_link_to('user_shifts', ['delete_shift' => $shift->id]),
                         icon('trash'),
-                        "btn-$class btn-sm border-light text-white"
+                        'btn-' . $class . ' btn-sm border-light text-white'
                     )
                 ]) . '</div>';
         }
diff --git a/src/Controllers/DesignController.php b/src/Controllers/DesignController.php
index db71dcc6..a9935f73 100644
--- a/src/Controllers/DesignController.php
+++ b/src/Controllers/DesignController.php
@@ -53,7 +53,7 @@ class DesignController extends BaseController
         $date = CarbonImmutable::now();
 
         for ($i = 1; $i <= 600; $i++) {
-            $formattedDate = $date->format("Y-m-d");
+            $formattedDate = $date->format('Y-m-d');
             $dateSelectOptions[$formattedDate] = $formattedDate;
             $date = $date->addDay();
         }