Fix typos in the specification markdown

Detected by https://github.com/client9/misspell

Author: TysonAndre

spec/07-variables.md

@@ -344,9 +344,9 @@ function f(){
   echo "\$fs = $fs\n";  // $fs = 
   $fs = 5;              // assign 5 to the function static $fs (local variable is not modified)
   echo "\$fs = $fs\n";  // $fs = 5
-  global $fs;           // define a global variabel with name $fs
+  global $fs;           // define a global variable with name $fs
   echo "\$fs = $fs\n";  // $fs = 
-  $fs = 3;              // assign 3 to the global variable $fs (function static and local variabel is not modified
+  $fs = 3;              // assign 3 to the global variable $fs (function static and local variable is not modified
   echo "\$fs = $fs\n";  // $fs = 3
   static $fs; 
   ++$fs;                // increment function static $fs

spec/08-conversions.md

@@ -4,7 +4,7 @@
 
 Explicit type conversion is performed using the [cast operator](10-expressions.md#cast-operator).
 If an operation or language construct expects operand of one type and a value of another type is given,
-implict (automatic) conversion will be performed. Same will happen with most internal functions, though some
+implicit (automatic) conversion will be performed. Same will happen with most internal functions, though some
 functions may do different things depending on argument type and thus would not perform the conversion.
 
 If an expression is converted to its own type, the type and value of the

spec/13-functions.md

@@ -17,7 +17,7 @@ case of an *outer function*, and an *inner function* defined within it.
 Until the outer function is called at least once, its inner function
 does not exist. Even if the outer function is called, if its runtime logic
 bypasses the definition of the inner function, that inner function still
-does not exist. The conditionally defined function comes into existance when
+does not exist. The conditionally defined function comes into existence when
 the execution flow reaches the point where the function is defined.
 
 Any function containing [`yield`](10-expressions.md#yield-operator) is a *generator function*.

spec/14-classes.md

@@ -1195,7 +1195,7 @@ The *method-modifiers* must not contain `static` and must define public visibili
 
 If the [dynamic property](#dynamic-members) designated by `$name` exists, this
 instance method returns `TRUE`; otherwise, `FALSE` is returned. The speficis of
-how existance of the dynamic property is determined is left to the implementor of the method.
+how existence of the dynamic property is determined is left to the implementor of the method.
 
 Typically, `__isset` is called implicitly, when the intrinsic [`isset`](10-expressions.md#isset)
 or intrinsic [`empty`](10-expressions.md#empty) is called with an argument that designates
@@ -1997,7 +1997,7 @@ __PHP_Incomplete_Class
 }
 ```
 
-Object of this class can be serialized, however, any attemt to call its method or access its property
+Object of this class can be serialized, however, any attempt to call its method or access its property
 for any other operation except serialization will result in a fatal error.
 
 ### Class `stdClass`