The problem:  you can't call ImagePSExtendFont a second time after calling it once then calling ImagePSText.

To reproduce:
// ...
// $f holds a loaded font, $im holds a loaded image
// and $black and white hold allocated colours
header('Content-type: text/plain');

$b = ImagePSExtendFont($f,1);
echo "<!-- ImagePSExtendFont returned $b -->\n";
ImagePsText($im, "Hello", &$f, 20, $black, $white, rand(0,100),rand(0,0100));
$b = ImagePSExtendFont($f,1);
echo "<!-- ImagePSExtendFont returned $b -->\n";

// ...

The second echo of $b will show false.

I picked thru the t1lib code and found that the ImagePSText code is creating size-dependent data associated with the font, which will cause subsequent calls to ImagePSExtendFont to fail (actually the underlying t1lib call to T1_ExtendFont returns the OP_NOT_PERMITTED error (#12)).

To fix:
You need to call T1_DeleteAllSizes before calling T1_ExtendFont in the imagepsextendfont function in ext/gd/gd.c.  This removes all size-dependent data which prevents the T1_ExtendFont from succeeding the next time:

/* {{{ proto bool imagepsextendfont(int font_index, double extend)
   Extend or or condense (if extend < 1) a font */
PHP_FUNCTION(imagepsextendfont)
{
#if HAVE_LIBT1
        zval **fnt, **ext;
        int *f_ind;
 
        if (ZEND_NUM_ARGS() != 2 || zend_get_parameters_ex(2, &fnt, &ext) == FAILURE) {
                ZEND_WRONG_PARAM_COUNT();
        }
 
        convert_to_double_ex(ext);
 
        ZEND_FETCH_RESOURCE(f_ind, int *, fnt, -1, "Type 1 font", le_ps_font);
	
	/* !!! I added this line !!! */
        T1_DeleteAllSizes(*f_ind);
 
        if (T1_ExtendFont(*f_ind, Z_DVAL_PP(ext)) != 0) RETURN_FALSE;
 
        RETURN_TRUE;
#else
        php_error(E_WARNING, "ImagePsExtendFont: No T1lib support in this PHP build");
        RETURN_FALSE;
#endif
}
/* }}} */      
              
After I added the T1_DeleteAllSizes, it worked fine.  Also, memory consumption decreases since a bunch of bitmapped stuff gets freed up.