diff --git a/devbox.go b/devbox.go
index 5df7574b432..2786084e86e 100644
--- a/devbox.go
+++ b/devbox.go
@@ -107,7 +107,11 @@ func (d *Devbox) Plan() (*plansdk.Plan, error) {
 		SharedPlan:      d.cfg.SharedPlan,
 	}
 
-	return plansdk.MergeUserPlan(userPlan, planner.GetPlan(d.srcDir))
+	automatedPlan, err := planner.GetPlan(d.srcDir)
+	if err != nil {
+		return nil, err
+	}
+	return plansdk.MergeUserPlan(userPlan, automatedPlan)
 }
 
 // Generate creates the directory of Nix files and the Dockerfile that define
diff --git a/devbox_test.go b/devbox_test.go
index 6c177b3ff1e..5d0bf75e1c8 100644
--- a/devbox_test.go
+++ b/devbox_test.go
@@ -71,6 +71,8 @@ func assertPlansMatch(t *testing.T, expected *plansdk.Plan, actual *plansdk.Plan
 	assert.ElementsMatch(expected.InstallStage.GetInputFiles(), getFileNames(actual.InstallStage.GetInputFiles()), "InstallStage.InputFiles should match")
 	assert.ElementsMatch(expected.BuildStage.GetInputFiles(), getFileNames(actual.BuildStage.GetInputFiles()), "BuildStage.InputFiles should match")
 	assert.ElementsMatch(expected.StartStage.GetInputFiles(), getFileNames(actual.StartStage.GetInputFiles()), "StartStage.InputFiles should match")
+
+	assert.ElementsMatch(expected.Definitions, actual.Definitions, "Definitions should match")
 }
 
 func fileExists(path string) bool {
diff --git a/examples/php/composer.json b/examples/php/composer.json
index b85759de98c..131e7589b99 100644
--- a/examples/php/composer.json
+++ b/examples/php/composer.json
@@ -1,11 +1,8 @@
 {
     "require": {
         "slim/slim": "^4.10",
-        "slim/psr7": "^1.5"
-    },
-    "config": {
-        "platform": {
-            "php": "8.1.10"
-        }
+        "slim/psr7": "^1.5",
+        "ext-mbstring": "*",
+        "php": "^8.1"
     }
 }
diff --git a/examples/php/composer.lock b/examples/php/composer.lock
index 6de3d5f29bf..63ff2e28c58 100644
--- a/examples/php/composer.lock
+++ b/examples/php/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "7d663301ad115500878a85269b118353",
+    "content-hash": "216dc58eec28d948d71d5fc808279ae3",
     "packages": [
         {
             "name": "fig/http-message-util",
@@ -770,5 +770,8 @@
     "prefer-lowest": false,
     "platform": [],
     "platform-dev": [],
+    "platform-overrides": {
+        "php": "8.1.10"
+    },
     "plugin-api-version": "2.3.0"
 }
diff --git a/planner/languages/php/php_planner.go b/planner/languages/php/php_planner.go
index 63e7c7eaea2..c65e582a0db 100644
--- a/planner/languages/php/php_planner.go
+++ b/planner/languages/php/php_planner.go
@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/pkg/errors"
 	"go.jetpack.io/devbox/boxcli/usererr"
 	"go.jetpack.io/devbox/planner/plansdk"
 )
@@ -50,6 +51,7 @@ func (p *Planner) GetPlan(srcDir string) *plansdk.Plan {
 			fmt.Sprintf("php%s", v.MajorMinorConcatenated()),
 			fmt.Sprintf("php%sPackages.composer", v.MajorMinorConcatenated()),
 		},
+		Definitions: p.definitions(srcDir, v),
 	}
 	if !plansdk.FileExists(filepath.Join(srcDir, "public/index.php")) {
 		return plan.WithError(usererr.New("Can't build. No public/index.php found."))
@@ -66,28 +68,33 @@ func (p *Planner) GetPlan(srcDir string) *plansdk.Plan {
 	return plan
 }
 
+type composerPackages struct {
+	Config struct {
+		Platform struct {
+			PHP string `json:"php"`
+		} `json:"platform"`
+	} `json:"config"`
+	Require map[string]string `json:"require"`
+}
+
 func (p *Planner) version(srcDir string) *plansdk.Version {
 	latestVersion, _ := plansdk.NewVersion(supportedPHPVersions[0])
-	composerJSONPath := filepath.Join(srcDir, "composer.json")
-	content, err := os.ReadFile(composerJSONPath)
+	project, err := p.parseComposerPackages(srcDir)
 
 	if err != nil {
 		return latestVersion
 	}
 
-	composerJSON := struct {
-		Config struct {
-			Platform struct {
-				PHP string `json:"php"`
-			} `json:"platform"`
-		} `json:"config"`
-	}{}
-	if err := json.Unmarshal(content, &composerJSON); err != nil ||
-		composerJSON.Config.Platform.PHP == "" {
+	composerPHPVersion := project.Require["php"]
+	if composerPHPVersion == "" {
+		composerPHPVersion = project.Config.Platform.PHP
+	}
+
+	if composerPHPVersion == "" {
 		return latestVersion
 	}
 
-	version, err := plansdk.NewVersion(composerJSON.Config.Platform.PHP)
+	version, err := plansdk.NewVersion(composerPHPVersion)
 	if err != nil {
 		return latestVersion
 	}
@@ -110,3 +117,49 @@ func (p *Planner) version(srcDir string) *plansdk.Version {
 	// might as well pick the latest version.
 	return latestVersion
 }
+
+func (p *Planner) definitions(srcDir string, v *plansdk.Version) []string {
+	extensions, err := p.extensions(srcDir)
+	if len(extensions) == 0 || err != nil {
+		return []string{}
+	}
+	return []string{
+		fmt.Sprintf(
+			"php%s = pkgs.php%s.withExtensions ({ enabled, all }: enabled ++ (with all; [ %s ]));",
+			v.MajorMinorConcatenated(),
+			v.MajorMinorConcatenated(),
+			strings.Join(extensions, " "),
+		),
+	}
+}
+
+func (p *Planner) extensions(srcDir string) ([]string, error) {
+	project, err := p.parseComposerPackages(srcDir)
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+
+	extensions := []string{}
+	for requirement := range project.Require {
+		if strings.HasPrefix(requirement, "ext-") {
+			name := strings.Split(requirement, "-")[1]
+			if name != "" && name != "json" {
+				extensions = append(extensions, name)
+			}
+		}
+	}
+
+	return extensions, nil
+}
+
+func (p *Planner) parseComposerPackages(srcDir string) (*composerPackages, error) {
+	composerJSONPath := filepath.Join(srcDir, "composer.json")
+	content, err := os.ReadFile(composerJSONPath)
+
+	if err != nil {
+		return nil, errors.WithStack(err)
+	}
+
+	project := &composerPackages{}
+	return project, errors.WithStack(json.Unmarshal(content, project))
+}
diff --git a/planner/planner.go b/planner/planner.go
index ed1f083d278..df3635c894e 100644
--- a/planner/planner.go
+++ b/planner/planner.go
@@ -67,15 +67,20 @@ var PLANNERS = []plansdk.Planner{
 	&zig.Planner{},
 }
 
-func GetPlan(srcDir string) *plansdk.Plan {
+func GetPlan(srcDir string) (*plansdk.Plan, error) {
 	result := &plansdk.Plan{
 		DevPackages:     []string{},
 		RuntimePackages: []string{},
 	}
+	var err error
 	for _, p := range getRelevantPlanners(srcDir) {
-		result = plansdk.MergePlans(result, p.GetPlan(srcDir))
+		result, err = plansdk.MergePlans(result, p.GetPlan(srcDir))
+		if err != nil {
+			return nil, err
+		}
+
 	}
-	return result
+	return result, nil
 }
 
 func IsBuildable(srcDir string) (bool, error) {
diff --git a/planner/plansdk/plansdk.go b/planner/plansdk/plansdk.go
index af13a266e34..30d504bb0ce 100644
--- a/planner/plansdk/plansdk.go
+++ b/planner/plansdk/plansdk.go
@@ -27,6 +27,8 @@ type Plan struct {
 	// application.
 	RuntimePackages []string `cue:"[...string]" json:"runtime_packages"`
 
+	Definitions []string `cue:"[...string]" json:"definitions"`
+
 	Errors []PlanError `json:"errors,omitempty"`
 }
 
@@ -118,23 +120,21 @@ func (p *Plan) WithError(err error) *Plan {
 	return p
 }
 
-func MergePlans(plans ...*Plan) *Plan {
-	plan := &Plan{
-		DevPackages:     []string{},
-		RuntimePackages: []string{},
-	}
+func MergePlans(plans ...*Plan) (*Plan, error) {
+	plan := &Plan{}
 	for _, p := range plans {
 		err := mergo.Merge(
 			plan,
 			&Plan{
 				DevPackages:     p.DevPackages,
 				RuntimePackages: p.RuntimePackages,
+				Definitions:     p.Definitions,
 			},
-			// Only WithAppendSlice the dev and runtime packages field.
+			// Only WithAppendSlice definitions, dev, and runtime packages field.
 			mergo.WithAppendSlice,
 		)
 		if err != nil {
-			panic(err) // TODO: propagate error.
+			return nil, err
 		}
 	}
 
@@ -142,7 +142,7 @@ func MergePlans(plans ...*Plan) *Plan {
 	plan.RuntimePackages = pkgslice.Unique(plan.RuntimePackages)
 	plan.SharedPlan = findBuildablePlan(plans...).SharedPlan
 
-	return plan
+	return plan, nil
 }
 
 func findBuildablePlan(plans ...*Plan) *Plan {
@@ -156,7 +156,10 @@ func findBuildablePlan(plans ...*Plan) *Plan {
 }
 
 func MergeUserPlan(userPlan *Plan, automatedPlan *Plan) (*Plan, error) {
-	plan := MergePlans(userPlan, automatedPlan)
+	plan, err := MergePlans(userPlan, automatedPlan)
+	if err != nil {
+		return nil, err
+	}
 	sharedPlan := &Plan{
 		SharedPlan: userPlan.SharedPlan,
 	}
diff --git a/planner/plansdk/plansdk_test.go b/planner/plansdk/plansdk_test.go
index 53b5a76c4e2..9f0499d84b0 100644
--- a/planner/plansdk/plansdk_test.go
+++ b/planner/plansdk/plansdk_test.go
@@ -24,7 +24,8 @@ func TestMergePlans(t *testing.T) {
 		RuntimePackages: []string{"a", "b", "c"},
 		SharedPlan:      SharedPlan{},
 	}
-	actual := MergePlans(plan1, plan2)
+	actual, err := MergePlans(plan1, plan2)
+	assert.NoError(t, err)
 	assert.Equal(t, expected, actual)
 
 	// Base plan (the first one) takes precedence:
@@ -51,7 +52,8 @@ func TestMergePlans(t *testing.T) {
 			},
 		},
 	}
-	actual = MergePlans(plan1, plan2)
+	actual, err = MergePlans(plan1, plan2)
+	assert.NoError(t, err)
 	assert.Equal(t, expected, actual)
 
 	// InputFiles can be overwritten:
@@ -80,7 +82,8 @@ func TestMergePlans(t *testing.T) {
 			},
 		},
 	}
-	actual = MergePlans(plan1, plan2)
+	actual, err = MergePlans(plan1, plan2)
+	assert.NoError(t, err)
 	assert.Equal(t, expected, actual)
 }
 
diff --git a/testdata/php/php8.1/composer.json b/testdata/php/php8.1/composer.json
index e69de29bb2d..bd4f794896b 100644
--- a/testdata/php/php8.1/composer.json
+++ b/testdata/php/php8.1/composer.json
@@ -0,0 +1,7 @@
+{
+  "require": {
+      "ext-mbstring": "*",
+      "ext-imagick": "*",
+      "php": "^8.1"
+  }
+}
diff --git a/testdata/php/php8.1/plan.json b/testdata/php/php8.1/plan.json
index 8b45032ee7b..da24852a706 100644
--- a/testdata/php/php8.1/plan.json
+++ b/testdata/php/php8.1/plan.json
@@ -21,5 +21,8 @@
   "runtime_packages": [
     "php81",
     "php81Packages.composer"
+  ],
+  "definitions": [
+    "php81 = pkgs.php81.withExtensions ({ enabled, all }: enabled ++ (with all; [ mbstring imagick ]));"
   ]
-}
\ No newline at end of file
+}
diff --git a/tmpl/development.nix.tmpl b/tmpl/development.nix.tmpl
index 4789c2075fc..9d6337ce13e 100644
--- a/tmpl/development.nix.tmpl
+++ b/tmpl/development.nix.tmpl
@@ -5,6 +5,9 @@ let
     url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
     sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
   }) {};
+  {{- range .Definitions}}
+    {{.}}
+  {{end -}}
 in with pkgs;
 buildEnv {
   name = "devbox-development";
diff --git a/tmpl/runtime.nix.tmpl b/tmpl/runtime.nix.tmpl
index fe686aa31c7..152d867a12a 100644
--- a/tmpl/runtime.nix.tmpl
+++ b/tmpl/runtime.nix.tmpl
@@ -5,6 +5,9 @@ let
     url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
     sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
   }) {};
+  {{- range .Definitions}}
+    {{.}}
+  {{end -}}
 in with pkgs;
 buildEnv {
   name = "devbox-runtime";
diff --git a/tmpl/shell.nix.tmpl b/tmpl/shell.nix.tmpl
index 05c642678b7..daec6bc5e78 100644
--- a/tmpl/shell.nix.tmpl
+++ b/tmpl/shell.nix.tmpl
@@ -5,6 +5,9 @@ let
     url = "https://github.com/nixos/nixpkgs/archive/af9e00071d0971eb292fd5abef334e66eda3cb69.tar.gz";
     sha256 = "1mdwy0419m5i9ss6s5frbhgzgyccbwycxm5nal40c8486bai0hwy";
   }) {};
+  {{- range .Definitions}}
+    {{.}}
+  {{end -}}
 in with pkgs;
 mkShell {
   shellHook =