Improve php string highlighting

Fixes #1753

runtime/syntax/php.yaml

@@ -12,7 +12,6 @@ rules:
     - special: "&[^;[[:space:]]]*;"
     - symbol: "[:=]"
     - identifier: "(alt|bgcolor|height|href|label|longdesc|name|onclick|onfocus|onload|onmouseover|size|span|src|style|target|type|value|width)="
-    - constant.string: "\"[^\"]*\""
     - constant.number: "(?i)#[0-9A-F]{6,6}"
     - constant.string.url: "(ftp(s)?|http(s)?|git|chrome)://[^ 	]+"
     - comment: "<!--.+?-->"
@@ -33,8 +32,12 @@ rules:
     - symbol.operator: "(=>|===|!==|==|!=|&&|\\|\\||::|=|->|\\!)"
     - identifier.var: "(\\$[a-zA-Z0-9\\-_]+)"
     - symbol.operator: "[\\(|\\)|/|+|\\-|\\*|\\[|.|,|;]"
-    - constant.string: "\"(\\\\.|[^\"])*\"|'(\\\\.|[^'])*'"
-    - constant.specialChar: "\\\\[abfnrtv'\\\"\\\\]"
+    - constant.string:
+        start: "\""
+        end: "\""
+        skip: "\\\\."
+        rules:
+            - constant.specialChar: "\\\\[abfnrtv'\\\"\\\\]"
     - symbol.brackets: "(\\[|\\]|\\{|\\}|[()])"
     - comment: "(^|[[:space:]])//.*"
     - comment: "(^|[[:space:]])#.*"